The diamond colour scale starts from D (The best blue White) to Z (for dark coloured diamonds).   Of all the diamonds that are suitable for selling in jewellery, only the top 8% have the colours D to G i.e. these are relatively rare. For white/colourless diamonds, the top colours: D (the Best Blue white), E (exceptional white), F (Very very white),G (rare white), H (white) and I (slightly offwhite) are all considered "white" when set in a ring and are hard to tell apart to the untrained eye: e.g. an E is hard to tell from a D, an F is hard to tell from an E, though the difference between say a D and an H may be more discernible when the diamonds are compared loose.   Professional grading labs judge colour from the side of the diamond not from the top.     Hence its impossible for even an expert eye to tell the difference between adjacent colours e.g. D, E and F for instance when viewed the top.   Colours from "K" show a slight "yellowing" and this effect increases as the diamond size increases.