The only reason i would take colossus up over domino is his immunities and armour ups. Damage wise domino and colossus are the same. Critical bleeds in lucky deal Godly damage.
Colossus can do freezer burn, caltrops, biohazard, is also immune against Incinerate (Mephisto, human torch) immune against armor break (Corvus, Medusa) and his armor ups make him a havok counter. One of the best well rounded champ's with utility and amazing damage. Even unduped, spam parry/heavy and he will drop opponents quickly. Add omega synergy and he goes to another level. Dupe him and he's on another. But even at his base, he is so powerful and more useful as a counter for specific types of content. My domino has been sitting at R3 for ages and don't miss her as I had other great bleeders like blade and Gwenpool who do that well too. Took colossus up over her and just did the same with havok for his niche utility and sp3 damage. Sunspot was my first R2 mutant as well and with so many mutants, really need to decide who is going to do the most for you in the long run. Domino trinity is fun and powerful, but not as useful in many scenarios. Ultimately go with what you feel will work the best for you though.