What ΔE means: putting a number on colour accuracy
"The match is pretty close" is an opinion. ΔE is the industry's way of making it a measurement: a number describing how different two colours look to a human observer.
The scale
| ΔE (2000) | What it means in practice |
|---|---|
| < 1 | Not noticeable to most observers, even side by side |
| 1–2 | Perceptible on close side-by-side inspection |
| 2–5 | Noticeable; reads as "very close" in normal viewing |
| 5–10 | Clearly different colours, same neighbourhood |
| > 10 | Different colours, no argument |
Why the formula got complicated
The first idea was simple: plot both colours in a "perceptually uniform" space and take the straight-line distance (ΔE76). But human vision isn't uniform — we're far more sensitive to changes in some regions (skin tones, neutrals) than others (saturated blues). ΔE2000, the current standard, corrects the raw distance with weighting for lightness, chroma and hue, plus an interaction term for the blue region. It's ugly maths with a clean goal: one unit of ΔE should look like the same amount of difference everywhere in colour space.
Interpreting accuracy claims
When a tool reports colour accuracy, three questions tell you whether the number is meaningful:
- Which formula? ΔE2000 numbers are not comparable to ΔE76 numbers — the older formula inflates differences in saturated regions.
- Mean or worst case? A mean ΔE of 2 with a 95th-percentile of 12 means most pixels match and some region is visibly off. Serious claims report both.
- Measured on what? Accuracy measured on the same images used to build the transform is honest for those images — it says nothing about unrelated ones.
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