Ok, according to the system there is 1 bar per 35 points. There are also brighter green bars for people over 500 points which ofcourse we haven't seen yet. There is a limit to 10 bars
can't remember the exact values, but the higher rank (post count) the rep giver has the more points is received. I think when a guru gives rep it's 3 points?
A mod is 5? And admin is more than mod
So yeah, can't remember it that well.
And the grey rep is from members that do not meet the requirements for giving rep. (too few posts, has just joined etc)