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Go to the beauty store and grab a bottle of 40 Volume Creme Developer. Use thin layer on parts that are Yellowing. Set in the sun and when it appears to be dry, add another layer.
You can watch videos on YouTube all day and people restore Yellowing on all sorts of plastics with this stuff. It will not damage other colors.







You can also try this guys procedure since it deals with action figures.
Moss Eisley: De-Yellowing Vintage Action Figures - Full guide