The colours range from D (colourless) and go all the way to Z (light yellow). 

A diamond's colour in reality actually refers to its lack of colour, thus grading the whiteness of the gem. The less colour a diamond has, the higher its colour grade. After cut, colour is generally considered the second most important factor while selecting a diamond. This is because the human eye tends to first detect a diamond's sparkle and then its colour.

DIAMOND COLOUR

D

Absolutely colourless: The highest colour grade. Extremely rare.

 

E-F

Colourless: Minute traces of colour can be detected by an expert gemologist.  Considered a rare diamond.

 

G-H

Near-colourless: Colour difficult to detect unless compared side-by-side against diamonds of better grades. An excellent value.

 

I - J

Near-colourless: An exceptional value with slightly detectable warmth or tone.