Diamonds come naturally in almost every color of the rainbow. However, the white range is the most popular. The color grading scale established by the GIA (Gemological Institute of America) varies from D (totally colorless) to Z (light yellow). Colorless diamonds are more valuable because they are rarer and their lack of color, or whiteness, in a diamond that allows the light to pass effortlessly through the stone and disperse that beauty back to the observer. D through F is referred to as "colorless" range. G and H are in the "near-colorless" category. I color diamonds appear white when mounted while J color diamonds face-up nearly white when mounted, but you can see some color in larger stones. Beyond M, most people can see color pretty easily. The color of the metal in a mounting can either mask or enhance the diamond color. Yellow gold makes slightly yellow or brown diamonds appear more colorless. White mounting (gold or platinum) makes the color more perceptible. #1 Diamond Source sells GIA graded loose diamonds with "K" or better color as well as "Fancy-Color" graded diamonds.
Fluorescence is not formally a color grading term. Many diamonds glow when exposed to light which contains relatively high amounts of ultraviolet. This is due to a natural interaction between the light's energy and the atoms in the diamond. Some diamonds (about 10%) fluoresce strongly enough so as to be somewhat noticeable in a regular (incandescent) light. Strong fluorescence in colorless to very near colorless grades (D through G) sometimes can give the diamond a hazy appearance. Generally, for very light yellow color diamonds, fluorescence is considered to be beneficial since it makes the diamond appear whiter. The beauty of any diamond that exhibits 'faint' fluorescence is not adversely affected in any way.