Diamonds come naturally in every colour of the rainbow. However most people are concerned with diamonds in the white range.  In other words, with the exception of some fancy coloured diamonds, the most valuable diamonds are those with the least colour.  The Gemological Institute of America (GIA) rates the body colour in white diamonds from D (colourless) to Z (light yellow).

The best colour for a diamond is no colour at all. A totally colourless diamond allows light to pass through it easily, resulting in the light being dispersed as the colour of the rainbow. Colours are graded totally colourless to light yellow. The differences from one grade to the other are very subtle and it takes a trained eye and years of experience to colour grade a diamond.