The only effective way to our knowledge, of yellowing plastic is to use a diluted wood stain. An alcohol wood stain can be diluted with water in a 1-to-5 ratio, or use a yellow stain.
UV and Heat
The way that most plastics naturally yellow is through ultraviolet degradation, and through heat degradation. Women with acrylic nails have complained about the yellowing effect of tanning beds on their nails.
There is no information out there on how to use UV or heat to yellow plastics. We may have to experiment with that at a later date.