Color is compared using letters D, E, F, G and so on. D is the whitest and rarest color and as the letters advance the color becomes a more tinted white.
Visually it is important to note that under most lighting conditions even G appears very white to the eye, so you may find that a H or I colored diamond suits your taste and budget.
Fancy colored diamonds (e.g. pinks, yellows and blues) have different grading systems.