Diamonds range from a total lack of color, D color on the GIA color grading scale, to ranges of colors in the yellow, brown, pink, and blue families.  The GIA color grading system begins at D and continues through Z.  Color differences are very subtle and it is very difficult to see the difference between, say, an E and an F.  Therefore, colors are graded under controlled lighting conditions and are compared to a master set for accuracy.  

Colors darker than Z are fancy colors. Fancy colors are generally more expensive due to the rarity of the stone.  Also, Fancy colored diamonds have their own color grading scale that runs from Light Fancy through Dark Fancy colors.  The intensity of the color will determine the desirability and the brightness of the color.