Green Rep- You've done well, here's a pat on the back, thanks for contributing to the community.
Red Rep- You've done not-so well, and have probably rubbed someone the wrong way. Don't do it again.
Grey Rep- Someone with insufficient reputation has given you a rep, but their's doesn't count because they have been disenfranchised due to their neg reps.
I'm not too sure on the grey rep, but I believe that's how it works. But in any case, it looks like you've made a positive contribution to the community, so a big HIGH FIVE to you!