Yes... It's time to stop smoking....
Don't expose your walk on sunshine too..
Sorry for that.... But I have many collection's toys since 20 years and I noticed something.... The toys that I clean up every month ( with coton soaked with water ) are not yellowed.... but the other yes... I'm sure it's the grease, the dust with the sun year after year.. It's yellowed the plactic... So clean up your plastic with light cleansing to preserve the plastic...
If the plactic is already yellowed.... sorry but it's too late... Time to make a custom....