Organic Steel Transformation: Colossus is a mutant with the superhuman ability to convert the tissue of his entire body into an organic steel-like substance. This substance resembling steel is of unknown composition but appears to be analogous to osmium and to carbon steel. He is able to transform into this armor-like state at will (the process is virtually instantaneous) and remain in that form for an as yet undetermined amount of time. (The longest time he has remained in armored form by choice so far has been five days). Once in his armored form he remains so until he consciously wills himself back to normal. If he is rendered unconscious, however, he spontaneously reverts to his normal form. While in the armored state, Colossus possesses the same degree of mobility that he does in his normal form. The conversion from flesh and bone to organic steel is accomplished by a psionic whole-brain interface with an ionic form of osmium, an extremely dense metal, located in another dimension. In willing the act of transformation, Colossus actually exchanges osmium atoms for his carbon atoms. The psionic interface with the other dimension re-creates all of Colossus's body in functionally similar organic ionic-osmium materials. The process by which Colossus gains additional mass from an unknown, perhaps extra-dimensional, force remains unknown. Colossus cannot become partially or selectively armored; his body is either entirely converted or not.
Juggernaut and Colossus have fought many times with Juggernaut winning but Colossus can hold his own. The only character who has caused physical damage to Colossus' body is the Green Scar as he bends Colossus' arm.
Juggernaut also fought Green Scar and lost but he wasn't at full power. After asking Cyttorak full power he was able to take it to Green Scar
Once the Juggernaut starts to move nothing can stop him unless it's a strong magical force.
Hulk uses that to his advantage and uses Juggernaut's own momentum to his advantage
Colossus fared better against Green Scar than Thing and all the others who fought him besides Juggernaut. This power upgrade should make things very interesting.