[Coco] Coco 3 - why just SOME yellow keys?

[Jeremy Michea]

I’ve been kicking around the idea of using one of those retrobrite formulas on a coco 2 and 3. I notice on Coco 3’s in particular that just certain keys turn yellow like ALT CTRL SHIFT F1 F2 etc. Did those keys use a different kind of plastic? Anyone have much success with retrobrite on a coco and if so, is there a good “how to” online? Can’t seem to find one. 

Also, now that winter is getting close, I’ve read there might not be enough natural UV light to make this work (unless one has an artificial UV light – I do not)