Both consoles have trademark (or signature) looks. In general, 360 is somewhat darker with a higher contrast. PS3 is slightly more washed out but generally clearer.

Hey look, halo 2 just went HD Nerds will probably correct me by explaining in great technical detail why the HDR lighting pixel vertex index shader direct 9 x 007 g-force yaddayaddayadda what have you not makes this games graphics so great... but don't bother, this to me (and many others) is just Halo 2....