Checking out Wiki and Marveldatabase, it seems as if little things outside of brute force and magnetism can affect this guy. So this thread is focused on who or what has affected Colossus. Just so people know, I don't need to know that a very powerful character like Galactus has affected him. That should be a given. 
 
Instead I recall the creator stating that Pyro has successfully heated up his metal skin before and I'm also aware that Omega Red's spores have affected him before. These are the type of things I want to know. So who or what else has affected him in the past?

" Checking out Wiki and Marveldatabase, it seems as if little things outside of brute force and magnetism can affect this guy. So this thread is focused on who or what has affected Colossus. Just so people know, I don't need to know that a very powerful character like Galactus has affected him. That should be a given.   Instead I recall the creator stating that Pyro has successfully heated up his metal skin before and I'm also aware that Omega Red's spores have affected him before. These are the type of things I want to know. So who or what else has affected him in the past? "

I don't know if she has, but can't Kitty Pryde affect him to some extent?????
Also if I recall hasn't Rouge taken his powers as well (I'm not sure I'm asking)
I would also thing that Vibranium would hurt him but once again I'm not really all that sure

i consider myself, and am by others a foremost on Colossus, so ill try and give you the rundown.
 
616 version
shown immunity
 
electricity/lightning
freezing (naturally!)
heat extremes (theres a limit though but its very high, ill get to this)
kinetics/pressure/force (again, theres  limit but its high)
other metals (at best, he can be scored scratched by adamantium, its a wash since when Colossus reverts
between forms nicks, cuts and scratches are repaired)
re-entry to a planets atmosphere 

magic. Colossus has shown some resitance to magic, in blast form a few times and spells.
this is largly due to enchantments his sister placed on him.

enough force can knock Colossus, and i often have seen those scans used as "effected", but
in almost every case the next page shows Colossus getting up
 
little to no resistance
magnetics
Legacy virus
telepathy (small degree of resitance.  the usual from having Charles and Jean poking you all the time)
certain specialized affects listed here 

* note: Colossus has undergone three evolutions. 
-a large portion of the time we've known him were referred to as his teen years. during this time he wasnt a master 
of his power yet. he had less strength, and could take some internal injury, usually to his bones if the outside
damage was great enough (teen)
* at some point here Colossus was said to have been upgraded by Magneto, but this was forgotten about quickly
by writers. it suppposedly increased his overall stats.
- due to long training with the X-Men and as an adventurer/superhero he evolved, and gained a mastery
of his powers. he was now much stronger, faster, agile, and notably able to assume a 100% metal form
straight through to his bones, much in the way Iceman uses his powers. (adult)
- Colossus of course died due to the Legacy Virus. he was brought back to life on Breakworld,
and his stats were said to be changed, but the how hasnt really been shown as this seems to
have been forgotten about (fine by me really, he shouldnt be made overwhelming)
 (reborn)
 
----
affected by but didnt really do anything
Wolverines claws scored him, not enough to due any harm nor put him down ( the Ultimates
version shows to not share this trait). (teen)
 
a Doombot remote controlled by Doom rendered Colossus unconscious using a  neural disruptor
to scramble his specific motor neurons made just for him (pretty damn crafty Doom!).
there were no lasting affects. Colossus's was asleep for a few minutes. (teen)
 
Rogues touch affects Colossus. shes never used it cause lasting harm in 616, but more than
likely could as its shown to be possible for her to do so. (any)
 
pressure point attacks as listed by someone above, but no lasting damage (any)

---
affected highly by but didnt do enough 

 Wreckers crowbar/ Asgardian enhancement= Colossus was still able to lift a ship weighing over 1,000 tons
while struggling with the injury. he was later healed by Zsaji  " in Secret Wars " in Colossus defense
he was fighting the rest of the Wrecking Crew and Dr.Octopus at the same time and Wrecker hit him
in the ribs from behind while his arms were lifted. (teen)
 
Worldbreaker Hulk was able to apply sufficient force to bend Colossus. Colossus healed from this.
i want to note that he did better than Thing here, and in fact Hulk complimeted him on his performance.
(current)
 
Vision affected Colossus by phasing and scrambling him. it took alot of effort on Visions part,
and resulted in a backlash KO'ing both characters. (teen)
 
Arkons arrows caused Colossus pain, no damage. Colossus was pretty angry. 
 
Pete Wisdoms daggers hurt Colossus. Petes daggers are hotter than the surface of the sun.
this was one of my favorite moments for Colossus because we learned alot from it. Wisdoms
daggers penetrated/melted into Colossus, and we saw his blood in metal form, which was
liquid metal as well. the affect was Colossus was extremely pissed and beat Wisdom into a coma.
he healed this by switching forms (adult)

---
affected by in a bad way 
 
Riptide's super velocity stars .
Magneto healed him but the process left him paralyzed temporarily.  
after he was fixed and improved, Colossus faced Riptide again
and was now immune to his stars.
Colossus ended up killing Riptide in the end .(teen)
 
Colossus took a blast from Doom after he had gained the powers of the Beyonder.
he was destroyed utterly. why this is notable (and debated) is when the heroes where
brought back, Colossus was noted as having had the greatest durability of them all,
having taken the least amount of physical damage. (teen)
 
---
hope this helps to answer your question.

Colossus greatest weakness for me is his big heart... once it brokens...he gets lost in his path. 

"Colossus greatest weakness for me is his big heart... once it brokens...he gets lost in his path.  "

" @Illyana Rasputin:  in 616, Colossus was unharmed by a chi attack from Iron Fist. in 616. Colossus has had 3 fights with Sym. at best Colossus knocked Sym out. alternate reality versions of characters are generally weaker than their  616 counterparts. the AoA version of Colossus for example was killed by a charged projectile throw to the back by AoA Gambit. "

Feel free to mention stuff from alternate universes too. Just curious on this guy's limits.

 Rogues touch affects Colossus. shes never used it cause lasting harm in 616, but more than 
likely could as its shown to be possible for her to do so. (any)     

" @Korg: debatable."

" @Korg: ??"

acid/corrosives and telekinesis
are prolly his biggest physical weaknesses

Rapid temperature change can be devastating to him in his metal form. Pyro superheated him and then Avalanche rolled a tanker truck full of liquid nitrogen into him, which severely cracked his skin and froze him into place. Even though it was a horrific moment for my favorite character, this is one of my all-time favorite images.

Kitty Pryde