I thought all the 500 ones were satin black unless they are made from carbon fibre in which case they would be gloss. I could be wrong though.

The Rouse style ones came finished in a gel coat and are gloss finish, I actually got my fibre glass Rouse style one re painted in satin/Matt black to give it the appearance of it being made out of rubber. I think splitters look better finished in matt black myself.

just before my car died and I broke it up and sold it in bits I fitted this carbon fibre Rouse style splitter, one of only two that were made out of carbon fibre at the time, gives you an idea of what a gloss finish looks like