Color difference is a common problem in coating workplaces.

The color is slightly different.
The customer says the color does not match.
It looked acceptable at the factory, but the customer rejected it.
Inspectors disagree.
The color difference meter says it is acceptable, but it looks wrong to the eye.

These issues are very troublesome on the coating shop floor.

Color difference problems are difficult because color is closely tied to human perception.
The same coated product may look different depending on the viewer, lighting, background, gloss, and distance.

Judging color only by a subjective sense of different or not different prolongs discussion.
The shop floor, quality assurance, and the customer can no longer judge by the same criteria.

Color differences should not be argued over on the basis of intuition.
They should be controlled using defined tolerances.

1. Color differences cannot simply be eliminated completely

First, completely eliminating color differences in mass-production coating is not easy.

Even with paint bearing the same color number, the actual coating color varies with many conditions.

Examples include the following factors.

• Paint lot
• Pigment dispersion
• Stirring condition
• Dilution ratio
• Thinner used
• Film thickness
• Underlying color
• Drying conditions
• Baking temperature
• Gloss
• Application method
• Inspection lighting
• Sample panel condition

It is therefore insufficient simply to regard color difference as something that must never occur.
In practice, acceptance limits must be defined on the assumption that some variation will occur.

These limits are the color difference tolerance.

Without a defined tolerance, even a small difference may be rejected.
Conversely, an excessively wide tolerance may fail to provide the appearance quality the customer expects.

The key is to define an appropriate tolerance for the product’s application.

2. Not every product needs the same level of color control

Stricter color control is not necessarily better.

High-level color control is needed, for example, for visible parts viewed at close range, products where brand color is important, and parts where the customer emphasizes appearance.

Internal parts, structural parts, and equipment parts seen only from a distance may not require such precise color matching.

Applying the same strict color control to every part causes the following problems.

• More inspection labor
• More touch-ups
• More recoating
• More waste
• Delayed delivery
• A greater burden on operators and inspectors
• Higher costs

This does not mean lowering the required quality.
The key is to differentiate color control levels according to the application.

For example, products can be classified as follows.

• High appearance control: appearance-critical parts viewed at close range
• Normal appearance control: general visible parts
• Function-first products: parts primarily intended for rust protection or other functions
• Approximate color permitted: internal and structural parts

This classification helps prevent both excessive quality requirements and insufficient quality.

3. Color difference control starts with an actual coated sample panel

An actual coated sample panel is the starting point for color difference control.

A color number alone does not adequately convey the appearance of the actual coating film.
Printed materials and PDFs are not final references for coating color either.

Use a sample panel made with the actual paint under conditions close to mass production as the reference.

An actual coated sample panel provides the following information.

• Actual color
• Gloss
• Surface texture
• Appearance at the relevant film thickness
• Effects of the underlying surface
• Changes caused by drying conditions
• The impression seen by the customer

Agree on this reference panel with the customer and manage it as the standard for mass production.

Create limit samples as needed.
A limit sample shows the boundary between acceptable and unacceptable.

Limit samples make visual judgments more stable.
They make acceptance criteria easier to share among operators, inspectors, and customers.

4. Color difference readings alone may not be sufficient

Color difference meters are sometimes used for color control.
They are useful instruments for checking color differences numerically.

However, judging everything solely by their readings is risky.

Numerical values do not always correspond exactly to the differences perceived by the human eye.

For example, even with the same color difference value, shifts in hue, lightness, or vividness affect perception differently.
Differences in gloss or surface texture can also make products look different despite similar color difference readings.

Even when using a color difference meter, the following conditions need to be defined.

• Measurement locations
• Number of measurements
• Measurement angle
• Measuring instrument
• Surface condition during measurement
• Permissible value
• Relationship to visual judgment
• Criteria agreed with the customer

A color difference meter supports judgment.
Its measurement method and acceptance criteria need to be standardized for shop-floor use.

5. Align lighting conditions for visual judgment

Color appearance changes with lighting.

A product that looks acceptable under factory fluorescent lights looks different outdoors.
It appears unacceptable in the customer’s inspection room.
Assessment at the Japanese head office gives a different impression from the local factory.

These situations do occur in practice.

Lighting conditions need to be aligned when judging color visually.

Check the following items.

• Which lighting is used?
• Is assessment under natural light or a standard light source?
• Is inspection-room brightness consistent?
• Is background color affecting judgment?
• Is viewing distance defined?
• Is viewing angle defined?
• Are gloss differences considered?

The same coating film looks different under different lighting.
Before discussing color differences, first check whether it is being viewed under the same conditions.

6. Background colors and surroundings also affect color appearance

Color may seem to be viewed independently, but it is actually affected by surrounding colors.

The same coating color may look different depending on surrounding colors, the background, adjacent parts, and the inspection table.

For example, a bright and a dark background create different color impressions.
Surrounding similar colors may make the coating blend in.
Conversely, nearby colors close to complementary colors may make a color difference seem stronger.

When comparing colors, surrounding influences should therefore be reduced as far as possible.

In practice, an achromatic background or mask is useful.
Achromatic colors have no hue, such as white, black, and gray.
They reduce the influence of surrounding colors and make the items being compared easier to assess.

Color difference control requires management of the viewing environment as well as the coating film.

7. Agree on color difference tolerances with the customer in advance

The most important step in preventing color difference problems is agreeing on tolerances with the customer in advance.

Responding becomes difficult if discussion starts only after mass production when the customer says the color is wrong.

Should the products be recoated?
Should they be sorted?
Should acceptance under concession be requested?
Should correction start with the next lot?
Who is responsible?

These issues then arise.

The following should therefore be agreed before mass production.

• Reference sample panel
• Limit samples
• Permissible color difference value
• Measurement method
• Visual judgment conditions
• Lighting conditions
• Viewing distance
• Checks when changing lots
• Standards for repeat production
• Sample panel update rules

Agreeing on color difference tolerances also helps preserve trust with the customer.

Unclear criteria make customers uneasy.
The shop floor becomes uncertain.
Quality assurance finds explanations difficult.

Color difference control is both technical management and management of customer agreements.

8. Color samples require storage control and updating

Reference panels and limit samples are not finished once created.

Sample panels can change color over time.
Light, heat, humidity, dirt, scratches, and storage conditions can change the reference itself.

When an old sample panel is used, mass-produced products may be judged defective even though the sample itself has shifted.

Sample panels therefore need storage and update rules.

Manage the following items.

• Creation date
• Approval date
• Paint used
• Paint lot
• Preparation conditions
• Approver
• Storage location
• Expiry date
• Update timing
• Comparison records with the previous sample
• Customer reapproval history

A color sample is the quality standard itself.
Color difference control will not stabilize if that standard is unmanaged.

9. Checks when a color difference problem occurs

It is premature to immediately suspect only the paint lot when a color difference problem occurs.

The paint lot may certainly be the cause.
However, many factors affect color differences.

First, check the following items.

• Is the reference sample panel current?
• Are the sample panel and production product compared under the same lighting?
• Is film thickness within the specified range?
• Is there a gloss difference?
• Are drying conditions as specified?
• Is the paint lot recorded?
• Was stirring sufficient?
• Is the dilution ratio correct?
• Are there differences in the underlying color or substrate condition?
• Are color difference measurement conditions consistent?
• Are customer and factory inspection conditions the same?

Organizing these checks makes it easier to distinguish color difference causes scientifically.

Subjective debates prolong color difference problems.
Organizing process and judgment conditions helps prevent recurrence.

10. Color control issues likely at factories in Vietnam

The following color control issues may be common at factories in Vietnam.

• No customer-approved reference sample panel
• Old sample panels that have not been updated
• Different lighting conditions at the factory and customer
• No defined color difference tolerance
• Visual judgment depends on inspector experience
• A color difference meter is available, but its measurement method is not standardized
• Differences in film thickness or gloss are mistaken for color differences
• No checking rules when paint lots change
• The same sample panel is not shared with subcontractors
• No cause-analysis procedure for color complaints

Under these conditions, each color difference problem becomes an ad hoc shop-floor response.

The key is to establish control criteria before problems occur, rather than judging differences afterward.

11. Organizing color difference control through 4M

The 4M framework also makes color difference control easier to understand.

Material: Materials

Check paint, pigments, thinner, substrate materials, and paint lots.

• Are paint lots different?
• Are there problems with pigment types or dispersion?
• Is the thinner compatible?
• Is the underlying color different?
• Has an alternative paint been used?

Method: Methods

Check application methods, film thickness, drying conditions, and measurement conditions.

• Is film thickness within the specified range?
• Is the dilution ratio correct?
• Is stirring sufficient?
• Are drying conditions the same?
• Is the color measurement method standardized?
• Are visual judgment conditions consistent?

Machine: Equipment

Check application equipment, ovens, lighting, and color measuring instruments.

• Are spray settings stable?
• Is there variation in oven temperature distribution?
• Is inspection lighting standardized?
• Is the color difference meter managed correctly?

Man: People

Check the judgments of operators, inspectors, and approvers.

• Does film thickness vary between operators?
• Do judgments vary between inspectors?
• Is color assessment training provided?
• Are criteria shared with the customer’s assessor?

12. Management perspective: Control color differences through a process

The Management perspective is very important in color difference control.

Stable color requires process-based control rather than reliance solely on shop-floor intuition.

Examples include the following systems.

• Rules for receiving color specifications
• Rules for preparing actual coated sample panels
• Customer approval records
• Reference and limit sample management
• Agreement on color difference tolerances
• Checks when changing lots
• Sharing samples with subcontractors
• Cause-analysis procedures for color complaints
• Sample panel update management

With this system, causes can be checked calmly even when color difference problems occur.

Manage color differences through a process rather than intuition.
This leads to stable mass-production quality.

13. Coating the Future with Science and Care

Color difference control is not intended to increase strict inspection.

Its purpose is to let the customer, quality assurance, and the shop floor judge by the same criteria.

Clear criteria remove uncertainty for operators.
Inspectors can judge fairly.
Explanations to customers become clear.
Unnecessary touch-ups and recoating can also be reduced.

This directly relates to CoatureVietnam’s principle of “Coating the Future with Science and Care.”

Science means controlling color differences through sample panels, measurement conditions, lighting conditions, and tolerances rather than intuition.
Care means reducing uncertainty and burden on the shop floor through that control.

Define tolerances suited to the product application rather than aiming solely for zero color difference.
This is practical color control that considers the shop floor.

Practical checklist

Check the following in your color difference control.

• Have color difference tolerances been agreed with the customer?
• Is an actual coated sample panel available?
• Are limit samples available?
• Are there rules for sample panel storage and updating?
• If a color difference meter is used, is the measurement method standardized?
• Are lighting conditions for visual judgment defined?
• Are viewing distance and angle defined?
• Are gloss differences being confused with color differences?
• Is film thickness variation checked?
• Are there checking rules when paint lots change?
• Is the same sample panel shared with subcontractors?
• Is there a cause-analysis procedure for color complaints?
• Are color control levels differentiated by product application?

CoatureVietnam’s perspective

Preventing color difference problems requires clear tolerances and control conditions rather than judging color by intuition alone.

CoatureVietnam supports color control at factories in Vietnam by organizing actual coated sample panels, limit samples, color difference tolerances, lighting conditions, and measurement conditions.

The purpose of color difference control is not to make everything more detailed and strict.
It is to create criteria that reassure customers, remove uncertainty on the shop floor, and allow quality assurance to judge fairly.