The rule is to contrast the socks with both the suit and the shoes. Thus if most of your suits are navy or charcoal, and most of your shoes are brown or black, acceptable colors of socks to purchase include: lavender, cerulean, honeydew melon, coral.
The pattern on your socks should always be at least as bold and discernable as the one on your next-most patterned piece of clothing (usually the ascot, or the undergarment).