Diamonds color is a quality of the stone that refers to the presence or absence of a yellowish color in white colorless diamonds. There are also colored diamonds, but this is something else entirely. Colored diamonds are very rare, and in most colors are actually more expensive because of their color. In diamonds that are supposed to be completely clear, color is undesirable but can exist because of the composition of the diamond. This quality never changes over time. Colorless diamonds allow more light to pass through them, just like a clear glass window, making them sparkle and shimmer. Utterly colorless diamonds are quite rare, meaning the whiter and clearer a diamond is, the more it is worth.
There is a grading system in place that rates diamond color. The color grade is represented by letters from D to Z. Colorless diamonds are rated D, E, or F, though diamonds with any of these ratings will appear identically colorless to the naked eye. Near colorless are rated G, H, or I. A faintly yellow diamond will receive a grade of J, K, L, or M. Very light yellow diamonds are graded N through R. Light yellow diamonds, which are quite visibly yellow, can receive a grade of S through Z. All diamonds graded D through H show zero or virtually no color and will present you with a gorgeous diamond to wear.
When considering which grade of diamond color to invest in, decide what setting you will use. If you choose platinum or white gold, a higher grade of color may be more desirable to look clean and crisp against the setting. For yellow gold settings, you can get away with a lower graded color because the gold color of the setting will obviously be much more yellow in color. This may not actually be a bad thing, either. While many people strive to buy the whitest diamond they can afford, some people prefer the warm glow of a faint yellow diamond.
There is another quality to diamond color other than the scale of clear to light yellow called fluorescence. This is an effect that can be seen in some diamonds when viewed under ultraviolet light. While gemologists tend to prefer diamonds lacking this effect, some people enjoy the glowing effect of their ring in such lighting. Regular lighting conditions do not expose this quality in the diamonds that have it, but the next time you are at a dance club, take a look at your diamond ring and see if you notice a fluorescent glow. While fluorescence is considered a flaw, and thus reducing the price of the diamond, the presence of fluorescence also causes diamonds to appear a bit whiter than they really are... the flaw becomes an advantage! You can buy a diamond which is cheaper because of both reasons - has fluorescence and a lower color grading and get a nice looking diamond for great price - a thing to consider.
Remeber that while Diamond Color is maybe the most known (along with diamond carat) of the Diamonds 4Cs, it is not the only one. Please read about the other Diamonds 4Cs as well - Diamond Carat, Diamond Clarity and Diamond Cut.