I can't help with taking the part off but there should be a way to do it. Was the discoloration from the sun or heat from the machine? If the yellow goes all the way thru the only way I know to restore the color is to paint the part. If it's just oxidized from exposure to the sun then you can sand the color out. I've had best results wet sanding plastic parts by hand. Sanding by machine generates too much heat and can (will) melt and smear the plastic. Start with a fairly aggressive wet/dry sand paper around 120 to 150 grit until the yellow is gone. Work to finer grits, say something like 120, 180, 220, 320, 400, 600, 1000. The grits of sandpaper may vary. The idea is that after you get the yellow out you sand with each finer paper until all the scratches from the previous paper are gone. Don't skip grits. It just takes longer if you do. It get easier as you go. Wash the part and change your water with every grit too so grit from the previous paper doesn't scratch the part. After you're done sanding you can buff it out with automotive rubbing compound to restore the shine. The last plastic I sanded was an old saw handle from the late 50s or early 60s. It took a couple hours to do. It's not a fast process. The other choice is to paint the part. If the yellow goes all the way thru it might be your only choice.
Rodney