If I remember correctly the"Grey Icon" is for people not eligible to vote yet.
IE-They don't have a high enough post count yet (50 I believe it's the starting point) to use the rep system (it was always like that on previous forums I frequented)
From OP
How does the reputation system work, and what limitations are there?
Everyone starts with 10 reputation points.
Members must have 10 reputation points to be able to affect other people's reputation. In other words - if you have -200 reputation points, you would have to earn back 210 points before you could vote on anyone.
Members must have 50 posts before being able to use the reputation system to vote on other people, however you can still earn reputation while under 50 posts.
Members can only click the reputation button 10 times per 24 hours. Choose wisely!
You must vote on 15 different users before giving reputation to the same person again (This is to help stop people voting up their friends for no reason!)
I believe the grey will change one they hit 50 posts.

