A diamond’s colour grade actually refers to the lack of
colour. In other words, diamonds that are white, containing
little or no colour, receive higher quality grades than those
with visible colour.

D, Absolutely colourless: This is the highest colour grade and is very rare.
E-F, Colourless: A rare diamond. Minuscule traces of colour can be detected
by a gemmologist.
G-H, Near-colourless: Colour difficult to detect unless compared with a
diamond of better grades.
I-J, Near colourless: The diamond can have a marginally detectable warmth
or tone.
K-M, Noticeable colour: An increasing presence in colour.
N-Z, Noticeable colour: Colour is obvious. These diamonds cost considerably
less in price.