Guides / Finishing

Two Black Parts, Two Different Blacks

Q: Can a supplier guarantee an exact colour match?

Not across separate batches, and no honest anodizer will claim it. What they can do is run a whole set together, keep parts from one material lot, and match to an approved physical sample within an agreed tolerance. That is the realistic ask.

Q: Is black more consistent than other colours?

Generally yes. Black uses a high dye concentration that saturates the oxide, so small variations in coating thickness show up less. Light colours, and especially clear and pale blues and reds, expose every inconsistency in the substrate and the coating.

Q: What about natural or clear anodize?

Clear anodize is not colourless. It takes on the tone of whatever is underneath, so alloy differences, machining marks and bead blast texture all show through. For matched clear parts, consistency in the machining and pre-treatment matters as much as the anodizing itself.

September 21, 2026 by The Alu CNC Parts editorial desk / how this is checked

A customer orders a housing and a lid. They arrive black, and the lid is very slightly browner than the housing under office light. Nothing was done wrong. They were anodized on different days.

Anodized colour is not paint from a tin. It is dye trapped in a porous oxide layer that the process grows on your specific part, and half a dozen variables feed into how that looks.

What actually controls the shade

The alloy. The single biggest factor. 6061 and 6082 dye evenly. 7075 runs darker and blotchier because of its zinc and copper. MIC-6 and cast alloys come out patchy. Two parts in two alloys will never match closely, whatever the anodizer does.

The material lot. Even within one alloy, two mill lots have slightly different trace chemistry and grain structure. This is why matched sets should come from one lot.

Coating thickness. Thicker oxide holds more dye and reads darker. A 12 um coating and a 20 um coating in the same tank on the same day will differ.

The surface underneath. A bead blasted face, a brushed face and an as-machined face will all look like different colours after the same clear anodize, because the oxide is transparent and the texture shows through.

Rack position and bath age. Current density varies across a rack. Dye baths deplete as they are used. These are the day to day variations an anodizer manages but cannot eliminate.

The temper. Heat treatment changes the microstructure the oxide grows on. A T6 part and a T651 part of the same alloy can read slightly differently.

What you can actually specify

Stop asking for a colour and start asking for a match to something.

Run the set together. The strongest single thing you can do. Tell the shop that a housing, a lid and four brackets are one assembly and must be anodized in one batch from one material lot. It costs scheduling, not money.

Approve a physical sample. Colour words are useless. “Matte black” means nothing across two continents. Have the shop send a first article, approve it, and write that production must match the approved sample. The sample becomes the specification.

Name a tolerance. For parts that matter, colour can be measured. Delta E is the standard scale, and a delta E of 1 to 2 is close enough that most people cannot see the difference side by side. Writing MATCH APPROVED SAMPLE, DELTA E < 2 is a real requirement a supplier can accept or decline.

Keep everything on one alloy. If an assembly has visible parts, do not mix 6061 and 7075 in it, and do not accept a 6082 substitution on half of them.

What to expect when you reorder

Six months later you reorder the same part. It will be a different material lot, a different dye bath and possibly a different anodizer. Expect a visible difference from the original run.

If the new parts have to live alongside the old ones, say so on the RFQ and send an old part as the reference. If they are a standalone batch, relax the requirement and save yourself the rework, because matching against a part made half a year ago is genuinely difficult and you will pay for it.

The specification that works

ANODIZE PER MIL-A-8625 TYPE II CLASS 2, BLACK, 15 um NOMINAL.
ALL PARTS IN ONE ASSEMBLY TO BE PROCESSED IN A SINGLE BATCH
FROM ONE MATERIAL LOT. COLOUR TO MATCH APPROVED FIRST ARTICLE.
BEAD BLAST ALL VISIBLE SURFACES BEFORE ANODIZING.

Four lines, and every one of them removes a variable. That is the whole trick.

Sources

Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.

Frequently asked questions

Can a supplier guarantee an exact colour match?

Not across separate batches, and no honest anodizer will claim it. What they can do is run a whole set together, keep parts from one material lot, and match to an approved physical sample within an agreed tolerance. That is the realistic ask.

Is black more consistent than other colours?

Generally yes. Black uses a high dye concentration that saturates the oxide, so small variations in coating thickness show up less. Light colours, and especially clear and pale blues and reds, expose every inconsistency in the substrate and the coating.

What about natural or clear anodize?

Clear anodize is not colourless. It takes on the tone of whatever is underneath, so alloy differences, machining marks and bead blast texture all show through. For matched clear parts, consistency in the machining and pre-treatment matters as much as the anodizing itself.

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