It's funny how many people forget Jenova's (or well, her cells anyhow) presence and influence in this game.
He didn't just forget about him cos he's forgetful. Jenova's cells were messing with his head. If I remember right there's a flashback in game: when he gets to Midgar he's already back in a similar state as he was when Zack helped him escape Nibelheim. Tifa finds him, the Jenova cells react off the memories from her of him AND what he expects and wanted of himself. He immediately snapped out of it and became Soldier Cloud with some of Zack's memories. He was still himself though; cold and unsocialble :P
As for the uniform colours, they messed it up. Simple. First class was black, second blue I think, and third *shrug* who knows. The creators/designers probably just changed it cos black looked cooler for a first class than bright purple (though the FF7 fmvs showed the uniform to be either a dark blue or purple *shrug*). Dissidia had Cloud back in a dark blue soldier outfit.