A particular type of coating color problem can be difficult for the shop floor to address.

The color looked satisfactory in the factory.
Quality assurance also accepted it.
It did not look noticeably different from the reference panel.

Yet, when it is checked at the customer’s site, the comments are:
“The color is different.”
“It is slightly red.”
“It looks yellowish.”
“It looks wrong outdoors.”

Such problems are not necessarily caused by the coating material itself.

One possible cause is a difference in light source.

Coating color is not determined by the film itself alone.
The same coated product can look different depending on the light under which it is viewed.

A product accepted at the factory may be rejected at the customer’s site.
Lighting conditions, the viewing environment, pigment composition, gloss, and background color may contribute to this.

1. Color becomes visible only in the presence of light

Color may seem to exist independently as a property of the object itself.
In reality, the color we see is determined by the relationship between light, the object, and our eyes.

Light strikes the coating film.
The film absorbs some of that light and reflects some.
The reflected light enters the observer’s eyes.
The observer then perceives color.

Even for the same film, changing the incident light changes the balance of reflected light.
This can change the perceived color.

Factory fluorescent lamps.
LED lighting.
Outdoor daylight.
The customer’s inspection-room lighting.
Dim warehouse lighting.
Evening light.
Direct sunlight.

These are not all the same light.
Different light sources can change the impression of the same coated product.

2. Factory lighting and the customer’s inspection-room lighting may differ

Coating factories often check color under the lighting installed in the factory.

However, those lighting conditions may not match the customer’s inspection environment.

For example, the factory checks under whitish LED lighting.
The customer checks under lighting of a different color temperature.
The Japanese headquarters checks under conditions resembling daylight.
The product is actually viewed in outdoor light during use.

With such differences, something that looks acceptable in the factory may look different elsewhere.

The important point is not to argue subjectively about which judgment is correct.
It is to agree in advance on the light source conditions used for acceptance decisions.

When a color difference complaint occurs, first check the following points.

• Which lighting was used at the factory?
• Which lighting was used at the customer’s site?
• Were the reference panel and mass-produced product compared under the same light source?
• Is the product intended for outdoor use?
• Are inspection conditions stated in the specification or agreement?

Comparing colors under different light source conditions does not allow a correct judgment.

3. Colors that match under one light may differ under another

Two colors can appear identical under one light but different under another.
This phenomenon does occur.

For example, 2 coated products look almost identical under factory lighting.
Outdoors, however, one looks slightly redder.
Alternatively, one looks yellowish in the customer’s inspection room.

This is more likely when the pigment compositions of the coating materials differ.

Even when the visible colors are similar, differences in the pigments used to create them may change their appearance when the light source changes.

Particular care is needed when using alternative coating materials.

Even when the same color number is matched, changing the manufacturer may change the pigments or resins used.
As a result, the colors may look similar under one light source but different under another.

When evaluating alternative materials, it is safer not to judge color under only one type of lighting.

4. The same color number does not guarantee the same appearance

Color numbers are useful for specifying color.
However, the same number does not necessarily produce the same appearance under every condition.

This is because the actual film appearance is affected by the following factors.

• Pigment type
• Pigment combination
• Resin type
• Film thickness
• Gloss
• Substrate color
• Drying conditions
• Surface finish
• Application method
• Light source
• Viewing angle

Solventborne and powder coating materials in particular may look different despite similar color specifications.
In powder coatings, particle size and mixing can affect appearance.
Metallic and special-effect colors may also change their impression with viewing angle.

Starting mass production using the color number as the only criterion can therefore lead to color difference problems later.

Color numbers are necessary.
However, actual coated reference panels must ultimately be managed together with the conditions used to check them.

5. A gloss difference can also make color look different

In a color difference problem, the actual influence may be a difference in gloss rather than the color itself.

The same color may look deeper or more intense at high gloss.
At low gloss, it may look whitish or muted.
A rough surface scatters light and changes the impression of color.

Even if a customer perceives a color difference, the cause may therefore be gloss or surface condition.

When checking color differences, also check the following items.

• Is the gloss the same?
• Is the film thickness the same?
• Is the surface finish the same?
• Is the substrate condition the same?
• Are drying conditions the same?
• Are the reference panel and production product coated by the same method?

Color and gloss are separate quality attributes, but they are strongly related in visual perception.

When investigating a color complaint, check gloss together with color.

6. Background color and surroundings also change color appearance

Color is also influenced by surrounding colors.

The same coated product may create different impressions against white and black backgrounds.
The color of an adjacent part or inspection table may also make it look different.

The assessment can also change depending on whether only part of a component or the complete product is viewed.

When comparing colors, it is important to minimize surrounding influences.

The following measures are useful in practice.

• Compare against an achromatic background
• Place the reference panel and production product side by side
• Use the same viewing distance
• Use the same viewing angle
• Check gloss differences
• Use a viewing mask when necessary

Correct color difference assessment requires a controlled viewing environment.

7. Compare reference panels and production products under the same conditions

A common problem in color checks is comparing reference panels and production products under different conditions.

For example, the reference panel is viewed in a clean inspection room.
The production product is viewed under lighting on the factory line.
The reference is a small flat panel, while the production part is curved.
The reference panel is new, while the production product has different storage conditions after drying.
Film thickness or gloss differs between the reference and production product.

Under these conditions, it becomes unclear whether the difference is truly in color or in the comparison conditions.

The following conditions should be matched when comparing colors.

• Light source
• Background
• Viewing distance
• Viewing angle
• Measurement location
• Film thickness
• Gloss
• Time since drying
• Surface condition

Color differences can be judged correctly only when the reference panel and production product are compared under the same conditions.

8. Also check the customer’s service environment

Color appearance also depends on where the product will ultimately be used.

For outdoor products, appearance in outdoor light matters.
For products viewed in stores or showrooms, appearance under that lighting matters.
For a component of machinery, the lighting at the installation location must be considered.

It is important to consider appearance in the actual service environment, as well as factory inspection conditions.

Particular care is needed for the following products.

• Parts with critical appearance requirements
• Products with brand colors
• Products for outdoor use
• Parts viewed at close range by customers
• Products used as an assembly of several components
• Repair products that must match existing items

Color matching should minimize the impression of mismatch in the environment where the customer actually views the product, as well as achieve factory acceptance.

9. Checking under several light sources is important for alternative materials

When adopting an alternative coating material, pay particular attention to appearance differences caused by light sources.

Even when existing and alternative materials are matched to the same color number, their pigment compositions may differ.
They may consequently look similar under factory lighting but different in daylight or the customer’s inspection room.

The following checks provide a safer approach to evaluating alternatives.

• Compare with the reference panel
• Prepare panels using the same application method as mass production
• Match film thickness
• Check gloss
• View under factory lighting
• View in daylight
• View under the customer’s inspection conditions
• Measure with a color difference meter when necessary
• Obtain customer approval

For alternative materials, simply having the same color number is insufficient.
The resulting film must be checked under several conditions.

10. What to check when a color difference complaint occurs

When a customer reports a color difference, first check comparison conditions instead of immediately suspecting only the material lot or a color-mixing error.

Check the following items.

• Which light source did the customer use?
• Which light source did the factory use?
• Is the same reference panel being used?
• Has the reference panel deteriorated?
• Is the viewing distance the same?
• Is the viewing angle the same?
• Is the background color influencing the result?
• Is there a film thickness difference?
• Is there a gloss difference?
• Is there a difference in drying conditions?
• Is the coating material lot recorded?
• Is an alternative material being used?

Deciding the cause without these checks may lead to inappropriate corrective action.

For example, changing the material will not solve the underlying problem if the cause is different lighting.
Correcting only the color mix will not help if the cause is gloss.
Suspecting the material lot will not resolve a difference caused by film thickness.

For color complaints, check the conditions that determine appearance as well as the color itself.

11. Organize color checks through 4M

Color difference problems caused by light sources can also be organized through 4M.

Material: materials

Check coating materials, pigments, resins, thinners, substrate materials, and alternatives.

• Do existing and alternative materials have different pigment compositions?
• Are there differences between material lots?
• Is the substrate color influencing the result?
• Is the intended gloss the same?

Method: methods

Check reference panel preparation, application conditions, comparison methods, and inspection conditions.

• Are panels and production products coated by the same method?
• Is the film thickness the same?
• Are drying conditions the same?
• Is comparison performed under the same light source?
• Are viewing distance and angle defined?

Machine: equipment

Check lighting, color measurement instruments, drying ovens, and coating equipment.

• Is inspection lighting standardized?
• Do the factory and customer have different lighting conditions?
• Are instrument measurement conditions standardized?
• Are oven temperature conditions stable?

Man: people

Check the judgment criteria of inspectors, operators, and the customer’s assessors.

• Do inspectors differ in their judgments?
• Are the same criteria used as the customer’s?
• Is training in color assessment provided?
• Are limit samples available for visual assessment?

12. Management perspective: agree on light source conditions

Preventing color problems requires agreement on light source conditions in advance.

For parts with critical appearance requirements in particular, it is useful to specify the following in specifications or quality criteria documents.

• Reference panel
• Light source for checking
• Viewing distance
• Viewing angle
• Background conditions
• Acceptable color difference range
• Acceptable gloss range
• Color measurement method
• Customer approval records

Clear definitions make it easier to align factory and customer judgment criteria.

Conversely, vague light source conditions leave uncertainty about whose judgment should apply when a complaint occurs.

Color difference control is not solely a shop-floor issue.
It is a management issue involving customer agreements, quality assurance, purchasing, coating manufacturers, and subcontractors.

13. Points requiring particular attention in Vietnamese factories

Vietnamese factories may have conditions that make light-source-related color problems more likely.

Examples include the following.

• Factory lighting differs between locations
• Inspection-area lighting is not standardized
• The customer’s inspection-room lighting differs from factory lighting
• The Japanese headquarters checks color in a different environment
• Outdoor products are checked only under factory lighting
• Alternative materials are assessed under only one light source
• The same reference panels and lighting conditions are not shared with subcontractors
• Comparison conditions are not recorded when complaints occur

Under these conditions, a product accepted at the factory risks rejection at the customer’s site.

Managing color requires managing the viewing environment as well as coating materials.

14. Coating the Future with Science and Care

The idea that colors look different under different light sources may sound somewhat technical.

For the workplace, however, it is a very practical issue.

A product is accepted at the factory but rejected at the customer’s site.
Operators are blamed each time.
Recoating or sorting is required.
Quality assurance struggles to explain the problem.
Sales personnel are consumed by customer support.

This places a substantial burden on the operation.

That is why matching color viewing conditions matters.
Clearly defining light sources, backgrounds, viewing distances, reference panels, and measurement conditions reduces uncertainty on site.

This is CoatureVietnam’s concept of “Coating the Future with Science and Care.”

Science means managing color through light source and comparison conditions, as well as subjective impressions.
Care means using that management to enable operators, inspectors, and customers to judge against the same criteria.

Accepted at the factory, rejected at the customer’s site.
Preventing this problem requires designing the color viewing environment as well as selecting coating materials.

Practical checklist

Check the following to prevent color problems caused by light sources.

• Is the light source for color checks defined?
• Do the factory and customer view color under the same conditions?
• Are reference panels and production products compared under the same light source?
• Are outdoor products also checked in daylight?
• Are alternative materials checked under several light sources?
• Are viewing distance and angle defined?
• Is the influence of background color minimized?
• Are gloss differences checked as well as color differences?
• Has the reference panel deteriorated?
• When a complaint occurs, do you ask about the customer’s checking conditions?
• Is inspection lighting standardized?
• Are the same panels and checking conditions shared with subcontractors?

CoatureVietnam’s perspective

Preventing color problems requires controlling color viewing conditions as well as coating materials.

CoatureVietnam supports color control in Vietnamese factories by organizing actual coated reference panels, inspection lighting, color difference checks, alternative material evaluations, and customer approval conditions.

Color is a quality attribute that becomes visible only under light.
Managing coating color therefore requires designing light sources and checking conditions as well as controlling the film.