Not sure if this belongs here or not.
Last summer I made a discovery about restoring Apple II/IIe/IIGS cases, and the cases on the peripherals.
I bought several significantly sun/light yellowed computers off ebay. I did not like the yellowing and searched the internet over and over looking for some way that I might bring back the original color to these computers and parts.
I stumbled upon using Hydrogen Peroxide as a whitener as a professional piano restorer was using this on piano keys. He was using a 30% solution which is not found at the local store, but 3% is. Not sure what 30% will do to plastic but the 3% works very well if given time.
I used the process on all the yellowed Apple cases that I have and it worked great, most you can't tell where there was yellowing to begin with. The one exception was a heat damaged case on a monitor. It is still a little brown.
I did not notice any whitening/lightening beyond the original case color. I see no reason why it would not work with most plastics. I have not tried it on significantly colored plastics, don't know what it would do.
How it works:
Hydrogen peroxide reacts with ultra-violet light to oxidize (among other things), how well it works depends on the wavelength and intensity of the UV light.
Note of warning, use rubber gloves on your hands and eye protection also. Apparently a by-product of this reaction on organic tissue is DNA damage. If nothing else you will get bleached skin/burns after a while (personal experience) as the peroxide soaks into the skin and oxidizes.
Here's how you do it:
1. Put cheese cloth or some other mostly transparent material over the case parts to be whitened/lightened.
2. Put the hydrogen peroxide onto the cloth.
3. Expose to sunlight or a good blacklight until dry.
4. Repeat steps 2 & 3 until you get the desired result.
Note: Bright sunlight works best. If left in the sun after peroxide has dried will yellow again. What worked best for me was to apply the peroxide liberally with a cotton ball so that the case was quite wet and I kept going out and reapplying about every 10 minutes for about 3-4 hours. using cheese cloth gives more time between applications, but also reduces the UV some.
Alternatively you can submerge the parts into the peroxide. You don't want them too deep as the hydrogen peroxide will absorb some of the UV passing through it. Also you want to cover it so it won't evaporate.