A diamond is like a prism. White light enters and divides into a spectrum of color. This spectrum of light reflects in colorful flashes of light called fire.
A diamond that contains color, from light yellow to brown, acts like a filter on the white light that enters it, and diminishes the spectrum of light that reflects out of the diamond.
So, less color is better. A diamond's color is determined by comparing it under controlled light to the Gemological Institute of America's color scale.
The most valuable and highly sought-after stones are those with no observable color. These stones are given a D rating on the color scale. This scale ranges from D to Z, as shown below. It is difficult to tell the difference between diamonds of one or two color grades in many cases.