When jewellers speak of a diamond's colour, they are usually referring to the presence or absence of colour in white diamonds. Colour is a result of the composition of the diamond, and it never changes over time.
Because a colourless diamond, like a clear window, allows more light to pass through it than a coloured diamond, colourless diamonds emit more sparkle and fire. The formation process of a diamond ensures that only a few, rare diamonds are truly colourless. Thus the whiter a diamond's colour, the greater its value.
NOTE: Fancy colour diamonds do not follow this rule. These diamonds, which are very rare and very expensive, can be any colour from blue to green to bright yellow. They are actually more valuable for their colour.
To grade 'whiteness' or colourlessness, most jewellers refer to GIA's professional colour scale that begins with the highest rating of D for colourless, and travels down the alphabet to grade stones with traces of very faint or light yellowish or brownish colour. The colour scale continues all the way to Z.
Which Colour Grade Should I Choose?
Diamonds graded D through F are naturally the most valuable and desirable because of their rarity. Such diamonds are a treat for the eyes of anyone. But you can still obtain very attractive diamonds that are graded slightly less than colourless.
What is Flourescence?
Fluorescence is an effect that is seen in some gem-quality diamonds when they are exposed to long-wave ultraviolet light (such as the lighting frequently seen in dance clubs). Under most lighting conditions, this fluorescence is not detectable to the eye. While most gemologists prefer diamonds without this effect, some people enjoy it. It's really just a matter of aesthetics.
This right up is from the GIA web site.