Assessment of colour is more than a numeric expression. Usually it's an assessment of the colour difference (delta) from a known standard. CIELAB (L*a*b*) and CIELAB (L*C*h) are used to compare the colours of two objects.
The expressions for these colour differences are DL*, Da*, Db*, or DL* DC* DH* ("D" symbolises "delta," which indicates difference).
Note: DH* is usually used instead of Dh to enable DE* to be calculated.
DH* = 2(C1*.C0*)1/2 sin (Dh/2)
Where C0* & C1* are chromaticities of the colours being compared and Dh is in the range +180°. (If Dh is outside this range then 360° is added or subtracted to bring it inside.)
Given DL*, Da* and Db*, or DL*, DC* and DH* the total difference or distance on the CIELAB diagram can be stated as a single value, known as DE*.
DE*ab = [(DL)2 + (Da)2 + (Db)2]1/2
DE*ab = [DE*)2 + (DC*)2 + (DH*)2]1/2
DL* = difference in lightness/darkness value (+ = lighter - = darker)
Da* = difference on red/green axis (+ = redder - = greener)
Db* = difference on yellow/blue axis (+ = yellower - = bluer)
DC* = difference in chroma (+ = brighter - = duller)
DH* = difference in hue
DE* = total colour difference value
Note: CIE stands for Commission Internationale de l´Eclairage (The International Commission on Illumination)