Duplicated Mesh, original rigs perfectly but copy goes all weird (jelly style)

Hey there, I’m a huge Blender newb. After struggling awfully to come up with an acceptable model, I’m trying my hand at rigging the initial character for a game. I made head, torso, right arm and right leg, each as a separate object, then duplicated and mirrored the leg and arm for the left side. When I applied the multiresolution modifier, I noticed that the duplicated arm and leg (the left ones) turned slightly darker than the rest of the mesh, but payed it no heed. Now it’s the second time I rigged, through a different tutorial and i’m still getting the same problem: The original arm and leg moves perfectly, bending like it should when I move the armature, but the left ones, which are darker, turn in a completely different way and seem to bend (or rather stretch) as if they were made of rubber. Since I can’t offer a better explanation than that, Here are some images to illustrate:

Of course I’m only assuming the fact that they’ve turned dark has something to do with why they’re behaving weird, but since the rigging was also duplicated I can’t understand why one side works perfectly and the other doesn’t.
both the arms and the legs are supposed to be in the same position, but the left ones turn out weird.
I appreciate any help with fixing this! Thanks in advance!

It looks like your normals are flipped. (It’s what is making your model greyish in color.) In edit mode select all of your vertices (Hot key-A), in the Tool Panel find Recalculate Normals. Click it. Done.

Good luck!

That solved it!
Thank you so much!