Hey Flynoob, welcome to the forum. Glad you found us. Feel free to make yourself at home here. And if you're just getting into fly fishing, keep asking questions-- lot's of helpful folks here and there's some good info in the FAQ section and old threads to browse through. (If you want some more pattern suggestions to round out your fly box, tell us what kind of fish you're chasing and the type of water you plan to fish.)
Although there aren't any "rules" when it comes to buggers or anything else in fly fishing you'll here all kinds of theories- some may even work occasionally. Some common ones are bright day = bright fly, dark day = dark fly. Big, heavy water = big fly, low water = smaller fly. Something that matches the common food items in general size and color is a probably a good choice, but sometimes flies in bright unnatural colors seem to trigger aggressive strikes.
Adding to what the others have said on buggers
Black-- looks like a lot of things including large stonefly nymphs, leeches, and minnows. Because it imitates a wide variety of things it's good in a bunch of different water types from rivers to lakes and ponds. It's also a good color for stained water for example, found in many streams early in the season or after heavy rains.
Olive- size 8, 10, 12 imitate damsel and dragon fly nymphs found in lakes and ponds. They might be a good choice to fish around weed beds.
(If I was just starting out, I'd pick a couple black buggers and a couple olive ones.)
Brown- in addition to crayfish it also imitates hellgrammites, so it's a good choice for smallmouth. It also imitates small minnows like sculpins and large stonefly and large dark mayfly nymphs.
White- imitates baitfish, and is also a very visible, bright color. To imitate baitfish, I'd probably go with a more imitative pattern for baitfish like a white Zonker, Black Ghost featherwing streamer or simple white bucktail or marabou wing streamer.
Although there are closer imitations for all of these different food items, buggers are often close enough to be decent imitations, as well as having some inherent characteristics of marabou that dances in the current, hackles that breathe and push water, and a profile that pulses on the retrieve and seems to suggest something real tasty.
There are all kinds of buggers with/without flash, rubber legs, coneheads, bead heads, and in all kinds of colors.
You might be interested in pics of buggers tied by forum members from our Woolly Bugger swap that Pocono ran a while ago:
Wooly Bugger Swap - the Flies / Part 1
Wooly Bugger Swap - the Flies / Part 2
They're so effective it's pretty hard to fish a bugger wrong-- and it may not even be possible. But here are some decent articles to give you some more ideas:
Fishing Woolly Buggers | MidCurrent
Streamers: Think Outside the Swing | MidCurrent