D colour is the highest grade and completely colourless (White). It is extremely rare, which translates a higher value.
E colour is considered colourless. It is only detectable by the gemologists when using master-stones for comparison.
F colour has a nearly undetected amount of colour. The difference between E and F is only visible if it is compared with D colour.
G colour is near colourless and the colour is only noticeable when compared to D and E colours.
H colour is near colourless, which will still appear white if not compared with higher colour grade diamonds.
I colour is slightly tinted, but not noticeable when the stones are mounted.
J colour is slightly tinted, but when it is set in yellow gold, the yellow tint in the diamond will be masked by the yellow gold.
K colour has a slight yellow tint that is noticeable.
L and M colours are similar to K colour. These diamonds have a yellow tint that can be seen with naked eye and still favoured by some.