I don't see how it would be a negative impact to the keg. Just make sure you mask off the locks so that you don't get any on them. Plus, leave enough of a clear area around the lid.
Personally, I'd rather get some Scotch Brite pads and give them a nice brushed finish. I did that to a couple of my sanke keg aging tanks (originally primaries, but I'm brewing slightly larger batches now). I did manage to clean up my one old corny keg pretty well before using it. It just takes some work. There's nothing saying it HAS to be a mirror finish.
BTW, if you do decide to paint them, then I would properly prep them first. That includes cleaning them, scuffing the surface, spraying on metal primer, and then painting with the finish. Just make sure the primer will work with the plastic-dip paint.