The more colorless a diamond the greater its rarity and value. The diamond color scale begins with D [whitest] and descends to Z [considerable yellow].

The GIA describes color as follows:

We recommend buyers select in the "D to J" range for two reasons: Whiter diamonds are simply more beautiful and the better colors have historically increased in value more rapidly.

Two diamonds with the same GIA color grades will not necessarily face up the same to the naked eye. Every diamond is unique and shows color and brilliance a little differently.

Interestingly, the same diamond submitted to the GIA for color grading more than once will not necessarily receive the same grade each time. How is this possible?

Color grade is judged on a spectrum and even though new electronic equipment can be useful, color is a subjective call and [within limits] it is understandable for a color grade to be seen slightly differently at different times.

In addition it is interesting to note that while the GIA is a worthy institution they certify any diamond submitted for grading and make no comment on value or brilliance.

A diamond selected in the D to H color range will not show any yellow tint... I and J colors will show just a hint. Buyers looking to own a larger stone [for the same money] can get very good value by selecting well cut I and J color diamonds.

Beautifully cut diamonds face whiter and brighter.