They might just key it up with some fine grit sandpaper and then prime it and re-coat it white... not sure if thats a bad thing, suppose it jsut means if you scrape it, it might show through red rather than black plastic... either way its probably going to be obvious if you scrape your rear bumper ;-)
Part of me says that spraying over the top of another colour is cheating but honestly i dont think the end result to you, the customer, will be any different, so long as they smooth out any imperfections in the original paint...