<--- Alpha.
I think he ups in rank... "Alpha" as a name is I believe applied to this one particular ARC, but "Alpha ARCS" are the first series... and they have a variety of ranks I guess, if I've read things right, and this one apparantly ranks up to "Captain" eventually, after his combat days, and is tasked to train Clone Commanders. I'm guessing his blue duds are his look for most of the war that he's active in combat, and his red is when he's promoted to Captain and kept at Kamino.
EDIT: Reading the rest of his Wiki article actually, that seems pretty much on other than he is in one last battle before he's wounded and disappears with Assajj Ventress after the war's pretty much winding down near (I assume) Order 66... Seems though that blue is just what they're all sporting, not just "Alpha" or "Alpha-17" as this Clone in particular is known... at least till they rank up to Captain or perhaps squad leader (Reds are also called "Sergeant" in the CW2D cartoon, which makes me think blue covers all the non-com ranks except for Sergeant, and from Sergeant up to at least Captain you're sporting red... No clue where the Yellow decoed ARC's from EU come into that unless they're above Captain).