Ironhide does have painted decoes and there was some discolouration on them.
(I also have Car Robot Ox, so RiD Ironhide is a more expendable figure that I used as a guinea pig)
I have bought H2O2 cream to try to use on figures to only apply the H2O2 to select parts, but then the weather in Sydney got gloomy so there haven't been any days with long enough sunlight for me to test it on. Might have to wait till next summer.

Any advice on what to do for parts that I don't want brightened but also want to protect from sun damage? Would regular sun block work to protect those parts?