A single number for how different two colours are.
A distance calculated in CIELAB. Several formulas exist — CIE76, CIE94, CMC and CIEDE2000 — and they give different values for the same pair, so a ΔE figure means little unless the formula is named.
There is no universal 'ΔE 2 is acceptable'. Tolerances depend on the formula, the material, the viewing conditions and the industry.
The same two blues can measure ΔE*ab 4.0 and ΔE00 2.0.
It is how colour matching is specified and checked in paint, print, plastics and textiles.
Both colours are measured, or calculated, as CIELAB values under the same illuminant and observer, and a formula turns the pair into a distance. ΔE*ab of 1976 is the straight-line distance. CIE94 scales the chroma and hue differences down as chroma rises. CMC(l:c), from the textile industry, does the same with a choice of weights, 2:1 for acceptability. CIEDE2000 adds corrections for lightness, for the blue region and for near-neutrals, and is the formula current print standards require. A stated tolerance therefore has four parts: the number, the formula, the illuminant and observer, and the reference it is measured from.
| Pair | ΔE*ab (1976) | ΔE94 | ΔE CMC 2:1 | ΔE00 |
|---|---|---|---|---|
| Cobalt Blue and Sapphire | 4.8 | 4.5 | 2.8 | 3.9 |
| Lemon Yellow and Citrine | 13.8 | 12.1 | 5.1 | 8.1 |
| Bone and Sand | 4.5 | 4.0 | 2.8 | 3.0 |
Lemon yellow and Citrine are 13.8 apart by the 1976 formula and 8.1 by CIEDE2000. The older formula treats CIELAB as uniform, and CIELAB exaggerates differences between strong colours. For the two off-whites, Bone and Sand, the gap is smaller (4.5 against 3.0), because the corrections shrink towards the neutral axis. A tolerance written as a bare 'ΔE 2' is a different tolerance in each column.
How this was calculated: All four formulas are applied to the CIELAB (D65) values of the catalogue HEX colours, with the first-named colour as reference; ΔE94 uses the graphic-arts weights.
A purchase contract says 'colour to be within ΔE 1' and nothing more.
Supplier and buyer use different formulas and different instruments, and each can prove the batch passes or fails. Print standards consider tolerances below about 2.5 ΔE00 impractical between different instrument models, so a limit of 1 may be tighter than the measurement itself.
A nearest-colour search ranks paints or fabrics by ΔE against a target.
The top result is the closest in number and may be the wrong way out: slightly greener where slightly darker would have been acceptable. The ranking is a shortlist to be judged by eye in the real light.