When jewelers speak of a diamond's color they are usually referring to the presence or absence of color in white diamonds. Color is a result of the composition of the diamond, and it never changes over time.
Because a colorless diamond, like a clear window, allows more light to pass through it than a colored diamond, colorless diamonds emit more sparkle and fire. The formation process of a diamond ensures that only a few, rare diamonds are truly colorless. Thus the whiter a diamond's color, the greater its value.
(Note that fancy color diamonds do not follow this rule. These diamonds, which are very rare and very expensive, can be any color from blue to green to bright yellow. They are actually more valuable for their color.)
To grade 'whiteness' or colorlessness, most jewelers refer to GIA's professional color scale that begins with the highest rating of D for colorless, and travels down the alphabet to grade stones with traces of very faint or light yellowish or brownish color. The color scale continues all the way to Z.
Diamonds are assigned color grades by comparing them to diamonds with color grades which have been certified by GIA as master color comparison diamonds. The color grade of a diamond is determined by arranging the GIA master color comparison diamonds about one half inch apart, the lightest on the left and the others on the right in order of increasing darkness To accurately determine color, a diamond must be completely free of all dirt and oil. A diamonds color is best observed when viewed through the pavilion in the table-down position on a flat white diamond grading trough with line of sight parallel to the girdle plane. A diamonds color grade is determined by moving it along the row of masters comparison guide diamonds until one seems to match. It is then placed on the side of the master, and then on the other side and compared carefully. If the diamonds color tone is lighter than the master, the next highest color grade is assigned. If the diamond is the same or only slightly darker, the grade of the master comparison guide diamond is assigned. To attain the most accurate analysis of a diamonds color it is recommended that a diamond be color graded in a white color free environment. Color grades are most accurate when evaluated in a darkened room using only the correct light source. Filtered, cool white, balanced fluorescent light is best; unlike sunlight, it is nearly free of ultraviolet light. The GIA GEM Diamond Lite is probably the best color grading light.