Using sunlight makes me think. There is a photographic process called "anthotyping" which involves coating a piece of paper with mashed up dyes from flower pedals, placing a sheet film negative over it (or some other pattern) and sticking it in the sun for a length of time. The result will be a photographic print.
Using UV rays to change the color of your plastic is the same thing; you could put a negative over it or some kind of pattern and bake it onto the guard in the sun.
Using UV rays to change the color of your plastic is the same thing; you could put a negative over it or some kind of pattern and bake it onto the guard in the sun.