Ferrous oxide, that's the ticket - self renewing, blends in to natural surroundings, biodegradable!
Oldphart - have you ever seen anyone going around sticking $100 bills on "pretty" motorcycles? I haven't either.
An old pal bead blasted his hardtail 1972 750 Honda chopper frame and clear coated it during a rebuild. After a few months it turned brown under the clear coat. It looked pretty cool on that old chopper, real "natural"... like it was being used regular.