The GIA Color Scale extends from D (colorless) to Z (light yellow or brown).  Although many people think of gem quality diamonds as colorless, truly colorless diamonds are actually very rare.  Most diamonds used in jewelry are nearly colorless with tints of yellow or brown.  Color grades are determined by comparing each diamond with a master set.  Each letter grade represents a range of color and is a measure of how noticeable a color is.   

D   Colorless E F G   Near Colorless H I J K                                      Faint Yellow L M N     Very Light Yellow O P Q R S       Light Yellow T U V W X Y Z