|Colorless||Near Colorless||Faint Yellow||Very Light Yellow||Light Yellow|
Colorless or white diamonds are more valuable than light yellow or brownish diamonds, within the range of the scale above, but darker colored stones, called "fancy", are more valuable than white stones. There is not a standard rating system in place for fancy diamonds, such as the prized blue diamonds.
The Gemological Institute of America (GIA) is a 75 year old non-profit organization that receives and grades diamonds from all over the world. They are the originator of the famous 4 Cs of diamond quality: cut, clarity, color and carat weight. Their D through Z color grading scale, and "flawless" through 13 clarity scale are recognized all over the world. They are the publisher of the quarterly journal Gems and Gemology.
See sample GIA report: DiamondGrading.pdf