The color of a diamond depends on factors such as pressure and temperature changes during formation, trace elements which may still be in the chemical structure of the diamond, and the presence of other elements, such as radioactive materials, during growth. Most diamonds range in color from white (colorless) to yellow. This is due to varying amounts of nitrogen within the diamond.  The most desirable and most valuable diamonds are the whitest or colorless diamonds.  The GIA has constructed the following rating scale to rank the color of various diamonds.   You should familiarize yourself with these ratings, or carry a copy of a chart like this when you are looking at diamonds.