Ok, now we are way off topic... your questions were something else I was going to investigate.. the whole brominated FR yellowing topic... not all plastics have Br flame retardants, some do. UV is the main culprit, it is tough on a lot organic materials, including people. Plastics yellow, they can also can be easily stabilized with additives. At the time when a lot of these parts were being manufactured, plastics industry was in a younger period than it is today, a lot was not known, and there was not 20 years of application history with components and resins.
Flame retardants add cost and kill mechanical properties - thats why they are not in everything... and also they are are terribly toxic. What is worse for you, the smoke from burning plastic or the gasses generated by things we put in the plastics to keep them from burning.
UV stabilizers work, they are expensive... so they were not always put into cheap toys. Not all plastics need UV stabilizers, some colors are more sensitive to UV as well, pigments can help with color stability.
I found this link - fear the man with 1 book.. but it adds some very dry wit that I have to admit I found hilarious..
https://www.quora.com/What-is-happening-in-terms-of-a-chemical-process-when-hydrogen-peroxide-is-used-to-restore-discoloured-LegoAlso, this guy speaks with a lot of chemistry knowledge and has a some experiments to back up his statements.
One of the reasons there are a lot of differences in how things yellow.. is that he parts that are made from "ABS" are remarkable NOT similar. They can have different pigment systems, different stabilizer systems (or no UV stabilizers), could be PC/ABS, or a different plastic entirely.
I have used this to clean legos, it works quite well, I used it on the Model M keycaps as well, just gives them a nice "pop" and very non aggressive
https://www.target.com/p/oxiclean-powder-versatile-stain-remover-free-3-5lbs/-/A-51980122#lnk=sametabThanks