Delta-E

Delta-E is a unit for the colour distance between two colours. The larger the number, the greater the “distance” between the colours. The main idea is that a Delta-E colour distance – in any direction – looks the same to the human eye. So if one colour is brighter by 5 Delta-E than another, to the human eye it looks just as distant from a colour that is redder or bluer by 5 Delta-E.

The most common method is Delta-E76 or CIE 1976 / CIE76.

This method uses the Euclidean distance, between the LAB colour values of the colour.

The calculation formula is: Square root of (L-distance squared) + (a-distance squared) + (b-distance squared)

Example: How far apart are the colours in Delta-E76: 
Colour 1: LAB 90/65/55
Colour 2: LAB 87/65/58

Calculation:
square root of [((90-87) = 3 squared) + ((65-65) = 0 squared) + ((55-58) =-3 squared)]
=
Square root of [(9) + (0) + (9)]
=
Square root of (18)
= 4,24

So CIE76 is quite easy to calculate, but the Delta-E76 calculation has its drawbacks, as it has been shown that the same Delta-E76 colour distance from the eye is not really seen as equally spaced, so newer calculation methods were established.

Other common methods are…

Delta-E CMC
Delta-E94 (1994)
Delta-E00 (2000)

Delta-E00 is currently the most common method used in proofing and printing technology, Delta-E CMC is widely used in textile printing.