Quality

Colour Approval for Phone Cases: ΔE, Master Samples and Why Phone Photos Fail

22 October 2026

The second order arrives, the navy looks purple, and the argument starts. In most of these cases nothing went wrong in production. The buyer approved a colour from a phone photo of a chip, and a photo is not a standard — it is a rendering of a standard through a camera, a screen and a colour profile that nobody recorded.

Colour has to be approved as a physical object under stated light, against a stated number. Here is how that works for moulded cases.

Three ways to specify a colour, in descending order of usefulness

1. Signed master sample. A physical chip in the actual material and finish, signed and dated by both sides, with a retain set held by each party. This is the only method that survives a change of supplier, a change of resin lot and a change of staff.
2. Pantone reference. Useful for communication, unreliable as an acceptance criterion on plastic, because Pantone chips are printed on paper and moulded TPU scatters light differently. A Pantone number is a starting point, not a standard.
3. Photo or screen reference. Not a standard. It is a mood board.

ΔE in plain language

ΔE is a single number describing how far apart two colours are in a perceptual colour space. It is not a percentage and it does not have an absolute meaning; what matters is the scale and the material. For moulded thermoplastics the working bands look like this:

Write the number into the purchase order. A PO that says "as approved sample" without a ΔE figure leaves the decision to whoever is holding the carton.

The three-light problem

The same case looks navy under daylight, blue-black under an office fluorescent tube and almost purple under warm LED. This is metamerism plus a genuine pigment effect, and it is why colour arguments often cannot be settled in a room with one light source.

Standard practice is to judge under three sources: D65 (north daylight), TL84 (European retail fluorescent) and A (incandescent). A colour that holds across all three is a stable colour. A colour that only holds under one is a colour that will generate a complaint from a customer standing in a different shop.

Matte, gloss and translucent: where tolerance gets harder

• Gloss reflects the light source directly, so the measured colour shifts with the angle of the instrument. Measure at the specified geometry, not handheld.
• Matte and soft-touch scatter light and read slightly lighter than the same pigment in gloss. A matte navy approved from a gloss chip will look wrong on arrival and be perfectly correct.
• Translucent and frosted are the worst case: the apparent colour depends on the wall thickness and on what is behind the case. Approve these on a finished part fitted to a phone, not on a flat chip.

The approval sequence that avoids disputes

1. Buyer sends a reference: Pantone, RAL, or a physical object.
2. Factory colour-matches and sends three physical chips per colour in the final material and finish, labelled with the batch and the date.
3. Buyer signs two, keeps one, returns one. The factory retains the third.
4. Both sides photograph the signed chip next to a grey card under D65 for the file — as a record, not as the standard.
5. Production is measured against the retained chip, and the measurement method is named in the PO.

What a deviation actually looks like in production

Most real deviations come from four causes, none of them dramatic: a different pigment lot from the masterbatch supplier, regrind content creeping above the agreed percentage, a barrel that was not purged between colours, and a change of texture grain on the mould. The first two are the frequent ones, and both are invisible in a photograph.

If a repeat order must match a previous one, say so in the PO and ask for the retained chip to be pulled and re-measured before the run starts. That one sentence prevents most of the conversations we end up in.

Material choice affects how stable a colour is over a product's life; the material comparison explains why silicone drifts and TPU does not.

Questions buyers actually ask

What ΔE should I put in a purchase order for phone cases?

1.5 to 2.0 under CIEDE2000 is a normal commercial tolerance for moulded thermoplastics. Tighter than 1.0 is achievable but raises the rejection rate and the price.

Can I approve a colour from a photo?

No. A photo is a rendering of a colour through a camera and a screen. Approve a physical chip in the final material and finish, signed by both sides, under a stated light source.

Why does my navy case look purple indoors?

Metamerism plus the light source. Judge under D65, TL84 and incandescent; a colour that only holds under one source will generate complaints in retail.