Most diamonds have a hint of yellow or brown. The rarest and most expensive colourless diamonds are D or Icy white - on a scale that goes to Z and is yellowish. (More colour than Z is graded as a ‘fancy' colour).
At Precious Metals all our diamonds are D to H and we advise you buy a diamond in that range because most people can easily detect the "off colour" in I and lower grades. However if you want the very best then we recommend icy white D to F "collection colours" as they are known in the trade.
However some people actually like the "warmth" of I ,J or K colours. But the main reason people buy a lower Colour is simply to trade up in one of the other 4 C's. For instance in Asia, low Clarity is synonymous with ‘impure' and so Colour is often traded off for a higher Clarity.
I find about half the people in a blind "Pepsi" taste test can tell the difference between a D and an H coloured diamond; at I and lower, the majority of people can see the faint tint of yellow.
Colour has a bigger impact on price as the clarity and carat weight goes up.
Glassy looking cuts like Asscher and emerald cut diamonds show more colour than ideal cut rounds; brighter cuts face up more colourless.
Now for something odd: It is easier to see the colour of a diamond in a white setting (platinum or white gold). If you have a yellowish diamond, then setting it in a rubbed over yellow gold bezel style setting can improve the apparent colour by a couple of grades.
Fluorescent diamonds with a bluish tint (about 30% of diamonds) usually appear more colourless