D Absolutely colorless. The highest color grade, which is extremely rare.
E Colorless. Only minute traces of color can be detected by an expert gemologist. A rare diamond.
F Colorless. Slight color detected by an expert gemologist, but still considered a "colorless" grade. A high-quality diamond.
G-H Near-colorless. Color noticeable when compared to diamonds of better grades, but these grades offer excellent value.
I-J-K Near-colorless. Color slightly detectable. An excellent value.
L-M Noticeable color.
N-Z Noticeable color.
Buying a VS1 versus SI1 diamond will not make much sense if both diamonds look the same without a 10x loupe in your hand.
A good bang for the buck is always the VS2, SI1 and SI2 graded diamonds.