Neverdull is actually meant for bare metal. I wouldn't make a habit out of using it on the stock rims. I used to get the same residue on my E1's. I think the discoloration is from the rust that forms on the rotors after they get wet. When you drive it, the wheels get blasted with the rust residue. I always used Meguiars Body Scrub, which took it off pretty easily. If not that, maybe try a mild polishing compound. I'd personally try to avoid painting the rims unless you're going to strip the entire finish off first. Every time I've seen someone try that, they always end up with a nasty, flaking paint finish.
