Delta E (CIE 1976)

The color difference, or $\Delta E$, between two colors ($L_1$, $a_1$, $b_1$) and ($L_2$, $a_2$, $b_2$) is:

$$\Delta E = \sqrt{(L_1 - L_2)^2 + (a_1 - a_2)^2 + (b_1 - b_2)^2}$$