Why did your SNES (or any plastic housing for that matter) turn yellow ochre? Vintage Computing and Gaming investigates the issue through good old-fashioned forum searching, Nintendo contacting, and expert chatting. An antiquated Nintendo response claims that the flame-resistant chemical used in the case discolors via exposure to heat, air, or light over time. One plastics expert suggested that manufacturers avoided using color defending plastics back in the day because of cost.
So what can you do to prevent plastic discoloration of your beloved SNES (assuming it isn't already hideous)? The article recommends coating it with spray-on plastic. That's right. Sleeve it.