Colour is the most argued quality in a paint shop, and most arguments come from judging colour in the wrong light. A part matched in the warehouse can look different in the customer’s booth and different again in store lighting. The light booth with an approved master and a dE measurement is how technicians make colour a number instead of an opinion.
This guide covers the colour-matching discipline: light sources, masters, dE tolerance and the batch workflow that keeps colours approved the first time.
Common Mistakes and How to Avoid Them
The pitfalls that show up most often in real projects, with the cause and the practical fix.
| Mistake | Why It Happens | Practical Fix |
|---|---|---|
| 1. Judging by window light | Metamerism arguments | Use the light booth with standard sources |
| 2. No approved master | Every batch is a new reference | Approve masters with the customer |
| 3. Old masters | Faded reference drifts approval | Date masters; store in the dark; renew |
| 4. Ignoring metamerism | Matches in D65, fails in TL84 | Check all three light sources |
| 5. dE measured on the wrong surface | Glossy vs matte reads differ | Measure on the same geometry and finish |
| 6. Colour checked before cure | Shade shifts after baking | Judge cured parts |
| 7. Film thickness ignored | Thicker film reads darker | Control build before colour |
| 8. No batch retention | Disputes without evidence | Keep a sample per approved batch |
| 9. Booth bulbs not calibrated | Light changes, judgements change | Calibrate and replace bulbs on schedule |
| 10. Different operators, different eyes | Approval varies by person | Use dE measurement, not just visual |
Best Practices That Hold Up in Production
The operating disciplines that separate a reliable line from a reactive one.
- Approve every colour against a master in the light booth
- Check D65, TL84 and CWF for metamerism
- Use dE <= 1.0 as the acceptance number
- Date and protect masters; renew on schedule
- Judge cured parts at the production film build
- Keep a retention sample per batch
- Calibrate the booth and bulbs on schedule
- Train operators to measure, not just look
Implementation Roadmap
A practical sequence that can be adapted to your own project.
Working Data & Formula Notes
Working data - colour control parameters
The annotations explain what each control protects, so a colour drift points to the right variable.
| Component / Parameter | Working Value / Role | What Changes Mean (annotation) |
|---|---|---|
| dE tolerance | <= 1.0 vs master | Above 1.0 the shade is visibly different; below is the working approval band. |
| Film thickness | Build per spec | Thicker film reads darker and changes gloss - control build before colour judgement. |
| Cure | Metal temperature per spec | Under- and over-cure both shift shade; judge cured parts only. |
| Light sources | D65, TL84, CWF | A match in one light and miss in another is metamerism - check all three. |
| Master storage | Dark, cool, dated | Faded masters drift approvals; renew on a fixed schedule. |
| Measurement geometry | Same angle and finish | Glossy and matte surfaces measure differently - standardise the reading. |
Reference Data
Specifications and references cited in this guide. Confirm final parameters with your line supplier.
Colour problem to check
| Matches in booth, fails on shop floor | Metamerism - check TL84/CWF |
| Batch darker than master | Film thickness high or cure shift |
| Batch lighter than master | Film thickness low |
| Approval varies by person | No dE measurement |
| Master looks different over time | Faded or damaged master |
Booth care schedule
| Bulb calibration | Quarterly |
| Bulb replacement | Per manufacturer hours |
| Background/geometry check | Monthly |
| Master renewal | Per schedule or on damage |
Implementation Cases
Case 1 - the shade that matched in the warehouse
Situation. A shop approved a colour at the bench, but the buyer rejected it in their booth; the part looked different under store lighting.
Approach. Colour approval moved to the light booth with masters and dE measurement, checked under D65, TL84 and CWF.
Outcome. Approval rate reached 100% because colour became a number, not an opinion, and metamerism was caught before shipment.
Case 2 - batch drift that tracked film thickness
Situation. A glossy red line drifted darker by the afternoon, and operators kept adjusting the paint.
Approach. Film thickness readings showed thicker coats in the afternoon. Build was controlled and colour re-checked on cured parts.
Outcome. Colour returned to the master without paint changes - the drift was thickness, not tint.
Frequently Asked Questions
Why does colour look different in different lights?
Metamerism: pigments respond differently to light sources. Check D65, TL84 and CWF in the booth.
What is a good colour tolerance?
dE <= 1.0 against an approved master is the working band for most hardware finishes; set it with the customer.
How do I create a master?
Produce a sample with the customer, approve it under standard lights, date it and store it in the dark.
Why did my batch get darker?
Film thickness rose or cure shifted. Measure thickness and judge cured parts only.
Do I need to measure dE or just look?
Measure. Eyes differ between operators and shifts; dE is the common language.
How often should booth bulbs be replaced?
Per manufacturer hours, typically with quarterly calibration. Old bulbs change judgements.
Why does my master look different over time?
Fading or damage. Store masters in the dark and renew on a fixed schedule.
Can I judge colour before curing?
No - shade shifts during cure. Judge cured, cooled parts at production film build.
What if the customer rejects a colour I approved?
Compare against the master and retention sample in the booth with dE data. The number resolves the argument.
Who can help set up colour control?
Paint process engineers can help define masters, tolerances and booth care - start with your finish list and failure history.
What Would You Like to Solve?
Colour arguments end when a master and a dE number exist. If you share the finish list and the rejection history, we can help you set the master, tolerance and booth workflow that gets colours approved first time.
Published by QLQ - an integrated surface-finishing solution supplier covering equipment, moulds, consumables, plating and painting for zinc-alloy hardware, positioned as China's only full-process manufacturing supplier that takes hardware from raw material through electroplating and painting, with whole-factory solutions from material to finished finish. Values cited are project references; confirm with your line supplier before specification.