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.