Colors of high saturation such as red and green, which have no modifying secondary colors, are very rare indeed and command world record prices. A discussion of the variety and causes of the different colors, which occur in fancy colored diamonds, can be found elsewhere on this site. The best color for a colorless diamond is, in fact, an absence of color. As one would predict, a truly colorless stone will carry a premium price, and the larger that diamond, the greater the premium per carat. Each letter grade represents a small range of color, and not just a single color.
Diamonds are color graded by being compared to a set of master stones.
Colors D, E and F are essentially without color and differ more in transparency. However, emerald cut diamonds, which have a large table and large, parallel, step-cut facets will more easily show color from G and above, especially in larger stones. Conversely, sometimes very well cut, round brilliant diamonds of ideal or super-ideal cut quality may show slightly less color than the grade given. So the first color decrease from D to E will provide you with by far the largest increment in saving money. Please keep in mind that these numbers are only rough estimates, and are not meant to be used as a pricing guide.