I was repairing an piece of older HP test gear, and the control buttons had all Yellowed with age and sun exposure to a large degree. So I pulled them all off the system, and tried the Hydrogen Peroxide + UV treatment. This process works great! You need 40% (or greater) hydrogen peroxide, which you can pick up from a place like Sally’s Beauty Salon, Clean the plastic, and then slather on the goop. Let it sit in the sun for 4 hours or so, and the plastic will return to it’s original color even if it’s a blue, brown or green etc.
“Before and After”