Color Distance (ΔE)

ΔE (Delta E) is the standard way to put a single number on "how different do these two colours look to a human eye". CIEDE2000 is the modern reference — values under ~1 are usually indistinguishable, under ~2 are very close, around 10 is clearly different, and 50+ is opposite-corner-of-the-wheel. CIE76 is shown for legacy use; RGB distance is included as a naive baseline.

The three numbers disagree on purpose, and the gap between them is the point. RGB distance treats the colour cube as ordinary geometry, so it rates a shift in the greens as no larger than the same shift in the blues, even though the eye is far more sensitive to the first. CIE76 fixes that by measuring in CIE LAB but still treats every region of that space as uniformly spaced, which overstates differences among saturated colours. CIEDE2000 adds the lightness, chroma and hue weightings plus the blue-region rotation term that were fitted to actual observer data, and it is the one to quote when a tolerance has to hold up.

Both inputs are read as sRGB and converted through linear RGB to CIE XYZ under a D65 white point. If your colours come from a wider gamut such as Display P3 or Adobe RGB, the same hex triplet describes a different colour and every figure here will be off. There is no chromatic adaptation step either, so results describe a D65 viewing condition rather than print or warm indoor light.

One thing ΔE cannot tell you is direction: a pair that differs only in lightness and a pair that differs only in hue can score identically, so a single value is a magnitude, not a description. It is also not a legibility measure. Text contrast is governed by the WCAG luminance ratio, which weights lightness alone, and a pair with a comfortable ΔE can still fail a contrast audit.