Diamonds are valued by how near they are to colorlessness - the less color, the higher the value. 
Color in diamonds varies from colorless to strongly tinted. The exception to this is fancy-color diamonds, such as deep yellows, pinks, and blues, which lie outside this color range.

 The colorless scale begins with the letter D, and continues with increasing presence of color to the letter Z. Each letter grade has a clearly defined range of color. Diamond color is all about what you can’t see. Pure or nearly pure diamonds are transparent. 

Colorless diamonds are extremely rare and very valuable. The presence of color is determined by impurities or structural defects naturally occurring in the rough. Many of these color distinctions are so subtle, but these slight differences make a very big difference in diamond quality and price. Diamonds are color-graded by comparing them to stones of known color, under controlled lighting and precise viewing conditions.