The GIA and EGL USA Color Grade is from D (colorless) to Z (brown or light yellow). Most diamonds on diamond jewelry are nearly colorless, they usually have small tints of yellow or brown. Colorless diamonds are very rare, the most expensive diamonds are D color or Icy white, diamonds that are more yellow than the Z color are called Fancy color diamonds. Fancy color diamonds are actually more expensive than most diamonds, sometime, they even fetch higher than the D color diamonds.
Diamonds are color graded by placing the diamond's table down and comparing the color to a master stones with special white lighting. Glassy looking cuts such as Asscher and Emerald cut diamonds show more color compared to ideal cut rounds diamonds. Brighter cuts faced up is more colorless and you can see their actual color when you look from the side of the diamond.
It is easier to determine the color of a diamond in a white setting such as a platinum or white gold ring. If you have a yellowish diamond, setting that diamond in a rubbed over bezel style setting can improve the apparent color by a couple of grades.
Some people prefer warmer colors between J - L on their yellow gold setting since it is not as apparent, it all boils down to saving money in order to get a higher carat weight or a better cut grade. Among the Diamond 4C's, color is a subject of preferences which can basically depend on the customer.
Some diamonds can emit a visible light when exposed to ultraviolet radiation, this is called fluorescence. Fluorescence is not a factor in determining color or clarity grades of a diamond. However, a description of a diamond's fluorescence strength is provided on EGL USA and GIA reports is present as an additional diamond characteristic.