olor is the "C" that you can see with your unaided eyes. The color grading scale developed by GIA (Gemological Institute of America) is the most widely accepted in the world and uses alphabet letters to indicate color. The letter "D" represents a total lack of color or "colorless," As you move down the scale (and the alphabet) through "near colorless," to faintly colored, you will see an increase in tint - usually yellow, brown or greyish tints.
A diamond can't be color graded face-up because the brilliancy (return of light to the eye by the facets) and dispersion (spectral colors) confuse the eye. Therefore color grading is done with the stone face down in a white card or tray and under proper illumination - a color corrected fluorescent light free of ultraviolet rays. This light is roughly equivalent to Northern exposure daylight.
Here is an analogy to think of: imagine a set of 23 glass tumblers on a window sill, each filled with distilled water. The first tumbler, or "D" tumbler is left untouched. Into each tumbler after the "D" tumbler, add an incrementally larger amount of tea, starting with just a couple of drops. By the time you get to the "Z" tumbler, the color of the tea will be quite obvious. However if you hold the "E" tumbler and the "D" tumbler side by side, it will be difficult to see any difference. The "H" or "I" tumblers will be much easier to detect a slight amount of tea tint as compared to the "D" tumbler again. This is very similar to color grading in diamonds.
As a diamond's tint increases it's value decreases. You will pay more for a "G" color diamond that an "H," however you may have a difficult time discerning the difference without a comparison.
Diamonds beyond "Z" have enough color that they are classified as "fancy," and their value increases. Some colors like the reds and blues, are extremely rare and therefore quite pricey. However, there are honey colors, browns, goldens and a whole group of colors in the champagne variety that can be quite beautiful and yet very affordable. The Canary is of course one of the more famous colors and is a pure, almost lemony yellow. While exceptionally beautiful, it too can fetch a "pretty penny."
Diamonds in the color range of D to J - colorless to near colorless - are the best to use in jewelry. If you want a rare diamond look for a diamond that is colorless (D-F). If you want to get a larger diamond for a comparable price get a near colorless stone (G-J).