I'm not sure what could be causing your shark to turn gray for an extended period of time. I've had my redtail for over 3 years and the only time I've seend him discolored is before the light turns on and when I've seen redtails at the store. So, I think their mood and lighting might affect their coloring (although when my light turns on, his tail is more pink rather than red) ... you might try testing your water, maybe he's signalling that something isn't right with your water.
As for being aggressive, i've read a lot of people having trouble with their redtails being aggressive and territorial. For the most part mine just goes where he wants as he is the biggest fish in my tank (almost 5") ... the only fish he bothers in my tank are my yo-yo's and he tends to only pick on the largest of the 3 yo-yos ... but they don't seem to bothered by it ... in the sense that he doesn't damage them and they don't run from him or avoid him unless he starts chasing them.
So, not sure how much help that may be ... but maybe check your water or change up his diet if it isn't eating normally. Until recently, I never saw my shark eat anything I added to the tank (I used to feed flakes, shrimp pellets & bloodworms almost exclusively) and I could never figure out what my redtail ate ... but he was growing fine, so I never worried about it ... but maybe try some tubifex ... mine will actually come out of his cave to eat it.