The standard color grading scale from the Gemological Institute of America (GIA) assigns alphabet letters to indicate color quality, from colorless to increasing tints of yellow that you can see unaided. Color is the most important factor to a diamond’s value.
The color grading scale developed by GIA is the most widely accepted in the world. The letter "D" is the purest value, representing a total lack of color, or "colorless." As you move down the scale (and the alphabet) through to near colorless, or “Z” (a faint yellow or brown), you will see an increase in the tint of yellow, visible with the unaided eye.
As a diamond's tint increases, its value decreases. Diamonds beyond "Z" that have color are classified "fancy" and their value increases. These are the blues, pinks, reds, etc. A diamond can't be color graded face-up because the brilliancy (the return of light to the eye by the facets) and the dispersion (spectral colors) confuse the eye.
Therefore, color grading is done face down on a white card or tray. For proper illumination, a color corrected fluorescent light, free of ultraviolet rays should be used. This light is roughly equivalent to North daylight. Diamonds that are colorless to near colorless are the best to use in jewelry (Colors D-J).
If you are looking for a rarer diamond, get a diamond that is colorless (D-F). If you prefer to get a larger diamond for the same budget, get a near colorless stone (G-J).