I sprayed mine years ago with undercoating, and they've stayed dark black since. I have used just regular black spray paint too after pressure washing them real good.
As long as you clean them real good, they'll be fine. I used the Wal Mart brand spray paint that's like .88 a can.
Just keep in mind if you use spray paint, it will most likely chip in the future and will need touching up. I still say use spray on Bedliner.