I got the beautiful German uncut DVD of Leone's official directing debut. In fact he had already directed most (if not all) of The Last Days of Pompeji (1959), whichwas credited to Mario Bonnard. Leone got an extra big mentioning as 2nd unit director.
Both show not much, or more correct nothing of the things to come. If you hope to find some of the brilliance or the trademarks of FoD, search on another place (maybe in Sergio's sand-box).
But when Pompeji is a simple Reader's Digest version of Quo Vadis (by pruning down the melodramatic parts and having much more action scenes plus an added earthquake, despite a much shorter running time), Colossus is at least a solid peplum with still plenty of action and intrigues.
TCoR is probably more orientated on the US films than on their Italian counterparts with a non muscle man hero and a less trashy story. Leone's directing is ... well, assured on a minor level, but nothing really special. Several nice scenes, absolutely no brilliance.
Let's say a 5/10
Corbucci's Romulus and Remus and The Son of Spartacus are the better films. And the more enthusiastic looking films.