yellow stained DUO-R

So called "simple" plastic will continually discolor as it's the cheapest plastic made.

You can get SNES clean by using a type of acid, but it is very toxic and will damage kidneys, so I won't name it here.

A combination of bleach, some bubbling denture cleaners like efferdent, and oxyclean will get almost anything white. Throw in an aquarium heater to accelerate the cleaning.

Usually not worth the trouble with snes. Duo, yeah. However exposure will just do it again in time. It depends on the blend of plastic they used.