Yeah, that error message is a bear. I fought it quite a bit customizing mine. Don't take my word for it, but IIRC, the problem I kept running into was that the filename already existed. Something on that order. I think what I did was make sure I renamed the files, or if I was bringing them over pre-made or from Photoshop, made sure the new name was different that the game-created so I could select it with confidence from the menu you're referring to in your OP. Remember though, if you select that file from that menu, you can't make changes in the color schemes from that screen any longer. At least w/o changing everything on that uni back to its original form.
Somebody with more skills than I possess will chime in, but rest assured it's nothing that you can't overcome with a little trial and error. Yes, it's probably harder than it has to be, but the game's worth it. You know it is.
