I have an ugly old cover to a bathroom fan that I needed to replace (and I lost the photos). Curious sort that I am, I decided to figure out WHY it turns yellow, which lead to a long path of chemistry and finally, a solution.
To make ABS plastic fire retardant, Bromine is added to it. With time and UV exposure, the Bromine reacts, but instead of reacting with hydrogen, it reacts with oxygen, and more and more of this reaction turns the plastic yellow and eventually brown.
What we need to do is to break the Bromine-Oxygen bond and make a Bromine-Hydrogen bond, basically turning back to its normal, un-yellowed state.
There are a couple of solutions. The one I tried with decent success was a container of a Hydrogen Peroxide and OxiClean.
1 x pint of 12% Hydrogen Peroxide (drug store stuff is about 3%, it’ll work but take days. Check with a hair care store for something 8-12%)
1 x Tsb OxiClean powder (grocery stores will have this in the laundry section)
Scale this as needed to submerge your plastic, i.e. 8 pints to a gallon, so 8 Tsb of OxiClean, etc.
Submerge your plastic and leave it exposed to the sun all day. If you need to brighten the back of it, turn it over and expose the back to sunlight the next day (keep it submerged).
The Solution II:
This is probably the easier solution.
1 x bottle of “40 Volume Creme Developer“, from your nearest beauty/hair salon. This is around $5, or ~$12 for a gallon jug!
Smear it all over your plastic, stick the part in a ziploc bag and let it bake in the sun all day (while turning it and massaging the bag to smear/spread the creme every few hours).
If the part is bigger than a zipper bag, use cling-wrap instead.