Reputation levels. When someone gives you a rep point by clicking on the scales at the bottom of the your post, they either add or subtract from the person's reputation level, depending on the reputation they give. Good Rep adds to the level, while bad rep subtracts from it.
The colors show how the person's doing. If they have green bars, they're getting a good reputation. The more bars, the better they are. Dark green shows a higher rep level for that good rep bar. If they have red bars, it means they have a bad reputation. The more red bars, the worse they are. A single grey or black bar means they have the reputation option turned off.
You really shouldn't hold someone to the reputation level under their name, though. It's become less useful now that people have abused it. Some people have given tons of bad rep to others who didn't like thier art or something. At the same time, though, some people are... how shall I put it... "betting" rep for contests. Meaning, the loser has to give the the winner good rep.
In other words, don't trust the rep system. It's become pretty much useless.