It's always better to sand a bit to "scratch" the paint so the other will stick more easily to the old coat, but some will tell you to go to the bare metal before priming it then color coat.
I too recommend to prime first but still you have to sand the old paint first.
So if you want to use UV inside,(uv reactive stuff and uv painted pieces in the case) the best color will always be flat black, cause it will accentuate your uv colors