I put a plastic pick guard (made by aNueNue) on a new matt finish Spruce top - had been on for a few months, decided I didn't like it/need it. When I took it off - there is a faint colour difference (As it is only faint (and a new top) it will no doubt fade to be totally indistinct. But at the moment you can see where it has been. It seems to have protected the top from light - and the non-covered area has deepened in colour slightly.
Ziegenspeck Spruce Tenor, Howlett Mahogany Concert, Alder Concert, Big Island Koa Concert, WOU Acacia Soprano, Hamano Mahogany Soprano, Acacia Sopranino, Tenor Cigar box, Risa Stick Soprano.