This is a shot of a fully deyellowed Apple IIgs keyboard.
I started off by washing all the plastics by hand with warm water and dish washing liquid. I then allowed everything to dry over night. I dipped the keys in a RetroBright liquid solution (3% H202), and I used the gel version of RetroBright (3% H202) for the larger plastics. One day outdoors (9AM to sundown) on a sunny Spring day was all it took to perform this magic.