Delta E Differences and Tolerances.

The difference between two colour samples is often expressed as Delta E, also called  DE, or ΔE. 'Δ' is the Greek letter for 'D'. This can be used in quality control to show whether a dyed or printed sample, such as a colour swatch or proof, is in tolerance with a reference sample or industry standard. The difference between the L*, a* and b* values of the reference and sample will be shown as Delta E (ΔE). The resulting Delta E number will show how far apart visually the two samples are in the colour 'sphere'

 CIE L*C*h°

This is possibly a little easier to comprehend than the Lab colour space, with which it shares several features. It is more correctly known as  L*c*h*.  Essentially it is in the form of a sphere. There are three axes; L* , c* and h°.  

The L* axis represents Lightness. This is vertical; from 0, which has no lightness (i.e. absolute black), at the bottom; through 50 in the middle, to 100 which is maximum lightness (i.e. absolute white) at the top.

The c* axis represents Chroma or 'saturation'. This ranges from 0 at the centre of the circle, which is completely unsaturated (i.e. a neutral grey, black or white) to 100 or more at the edge of the circle for very high Chroma (saturation) or 'colour purity'.

The h* axis represents Hue. If we take a horizontal slice through the centre, cutting the 'sphere' ('apple') in half, we see a coloured circle. Around the edge of the circle we see every possible saturated colour, or Hue. This circular axis is known as h° for Hue. The units are in the form of degrees (or angles), ranging from 0° (red) through 90° (yellow), 180° (green), 270° (blue) and back to  0°. 

The Lch colour model is very useful for retouching images in a colour managed workflow, using high-end editing applications. Lch is device-independent.