I'm pretty sure you have to win 25 times with the character you're trying to unlock the colour with. Just set up a second controller in idle and play some quick one-round matches in versus. I don't think it matters if an actual person is using the other controller.
I haven't tested it myself, but just try it anyways. It might work.
"Yeah, cause' what do you do in a mansion full of zombies? Play Moonlight F****** Sonata." --random youtube comment