[Stuart]

There's some info on Oyster White on page 6 of this paint chip chart, although I don't know how well the codes shown there translate to modern products. https://www.autocolorlibrary.com/pages/1980-GM.html

I've read about a method of whitening up old yellowed plastic using hydrogen peroxide and a UV light source. There are also other methods using vinegar, bleach and other products too. If you do a google search on 'yellowed plastic fix' a number of these show up, it may be worth investigating.