This is the colorless diamond group. They are common enough to be found in any store, but still command the highest prices. D and E show virtually no color.
F starts to show the very slightest of color and is the transitional color to the next group.
This is the near colorless group. If you are looking for value when you shop, ask to see stones from this group. These diamonds begin to exhibit a very light shade of color, but it is hard to detect in mounted stones. This is the group that begins to show slight yellow color which may be noticeable from the top of the stone and even more from the sides. Diamonds in the N-R group show yellow color even when mounted in jewelry.