That's exactly what makes Colossus' costume less ridiculous then Superman's or Batman's because he's not wearing underpants over his pants. My guess that this was originally done was to show more of his metal skin with the purpose of looking more powerful or imposing.
Technically, during some of the early comics, Colossus actually wore some blue tights over his thighs that would vanish when he transformed, although that could have simply been a colorist's way to hide his man-gams during the 70's. That looked stupid too because blue wasn't and still isn't part of his costume scheme.
NO MAN looks more imposing showing his thighs while wearing knee high boots. At least no one outside of THE VILLAGE PEOPLE.
Other superheroes like Batman, Superman, etc. have outdated costumes because they are too iconic to change for very long. Batman got the all-black look after the first two movies but that switched back around 1998-1999 or so. And Superman faces *****ing if he dares to tweak his S-shield a little; imagine if he decided to find a way to undo the underoos.
Colossus doesn't face that sort of iconic status to his costume. It has been tweaked and altered many times. The perennial theme seems to be to not wear pants, which is odd because it also doesn't match his character at all, really. He isn't flamboyant or anything. He doesn't strut his bod around to score with ladies too often. He's usually more modest. The only way it makes sense is if you compare Colossus' costume to that of wrestlers', but he isn't a wrestler type, either.
In comics, if you are a man who doesn't wear pants, you're either living underwater or a sidekick. Or one of those cosmic types with clashing colors who can say something omnipotent like,
"My colors clashed before there WAS color!" and you can't argue it. Colossus isn't a sea dweller, a sidekick, or a cosmic, damn it. Underoos over tights or whatever, pants are pants.
It just seems that because Cockrum designed the original look, no one wants to majorly F with it to give him pants on a regular basis.
I even made a sprite comic (using sprites from Scrollboss.com/PrimeOp's website) regarding the topic months ago.
So, hopefully now that Colossus is randomly deciding to branch out, slap on some pants, and, um, hopefully Casey can figure out some way to make proper use of him when She-Hulk is on the team and she can do everything he can do better anyway.
INCLUDING not having to wear pants.
