Ultron:
So while fighting Ultron Omega in the quest to unlock colors I noticed a property that his heavy Protonium Emitter (qcb+hp) has, and that’s while the orb is actively hitting the enemy, whether hit stun or block stun, it freezes time for that particular enemy. So in the case of Ultron Omega you can just repeatedly use Protonium Emitter freezing him in place and he can barely move for a second before the next one comes out. I use hp, qcb+hp repeatedly, hp for damage and qcb+hp to freeze time.
The interesting part is that Ultron Omega has three forms, attacking mode, blocking mode and drone mode. Using this method you can essentially freeze him in place during his attacking form until he retreats and puts up his fists, but the trick is to just keep on doing it and he will do absolutely nothing besides occasionally retreating into drone form, after which killing the drones he comes back out and you repeat the process again until he is dead.
If you’re only after colors for certain characters it might just be simpler to do arcade mode with Ultron/Whoever and not have to mess about trying to beat Ultron Omega with characters you don’t know. This is pretty much fool proof, the biggest danger really is the drones, but if he does attack you just roll forward and continue the method.
It will take a while for everyone to get each colour but if you’re only after a few this might be the way to do it.