Green is positive, Blue is neutral, Red is negative.
When I first started the reputation system, I forgot to give starting points (10) to everyone. Without starting points, everyone had 0 rep (blue icon) except a few people (ill explain later).
As you gain rep, you will gain more green rep icons, and you lose rep you will gain red icons, and if your neutral -9 to +9 you will have a single gray icon.
The more rep you have the higher impact you have when you rep someone. For example someone with 10 rep can increase (or decrease) someones rep by 1 point. Someone with 1000 rep will increase (or decrease) a users rep by 10 points.
Two ways to gain rep; by getting rep from someone else, and by posting.
When I started the rep system, everyone had 0 rep except admins and users with a high post count. That's why some where gray and some were green. I forgot to give everyone 10 starting rep, because with 0 or less rep, you can not affect someones rep.