How different are two colours, really

Not how far apart their hex codes are — how far apart a human eye puts them. That is what CIEDE2000 measures, and the two answers disagree most exactly where it matters.

First colour

Second colour

ΔE00 — perceptual distance

1.12

barely distinguishable

Between 1 and 2 — a trained eye spots it when the two touch; nobody spots it across a room.

Plain RGB distance, for comparison

—%

Straight-line distance between the two hex codes, as a percentage of the largest possible. It has no relationship to what you see — which is the point.

The example that shows why this matters

#ff0000 and #ee0000 differ by 17 in one channel, and come out at ΔE00 3.64 —visibly different. #0000ff and#0011ff differ by the same 17 in one channel, and come out at ΔE00 1.12. Same arithmetic distance, different answer, because the eye is far less sensitive to change in the blue corner than in the red.

Both numbers on this page were computed by the same function the build uses, which is checked against the 34 reference pairs its authors published.How that check works.