The general rule is that a gentleman's socks shall match the colour of pants he's wearing. It's fine to have patterning on it, or somewhat different material, but the colour should match (or close). My problem now is that I have socks that are close, but for some pants, the colours are off and I have nothing to match. So I think I will try to buy socks at the same time as my pants.

One exception (self-declared) to this is my jeans. I just wear colours that match the rest of my clothes (ie. shirt / shoes) or that seem to extend the general style of outfit that I have that particular day. With my bowling (-styled) shoes, I tend to wear different colours to bring out the colourful shirts / other clothes I'm wearing. Like Ben in Technicolor.

Belts should match the shoes, regardless. If my shoe has a little buckle, I also try to make sure the belt does also. It basically leads to belt-shoe sets that I swap between, runners/sneakers excepted.