This yellowing happens thanks to a flame retardant called bromine in old ABS plastics. When exposed to UV light, the bromine molecules can destabilise and leech through to the surface, causing the plastic to turn yellow (or even brown if left long enough). Modern plastics have improved the chemistry so this process doesn’t happen.
Different items will turn yellow at different rates than others, even from the same line of products. Your wall controller in one room may be much yellower than your other controller in another room, just because they were from different batches of plastic. Sometimes, two pieces of plastic in the same machine can be different levels of yellow which is due to the different compositions of bromine in the various plastics.
Unfortunately there is no real way of stopping this from happening or reverting the plastic back to the original colour that we can recommend.