ΔE (delta E) is a number that expresses the difference between two colours, calculated from L*, a*, b* values measured with a spectrophotometer. The lower the value, the closer the colours. There are several formulas (CIELAB, CMC, CIEDE2000), so a value should always be quoted with the formula used.
As a rough guide, below 1 the difference is barely visible to a trained eye, and around 2 to 3 it becomes noticeable. In textiles ΔE CMC 2:1 is widely used. ΔE measures the size of the difference but not its direction, so shades are also checked visually under standard light.
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