Darker Corian colors do not lend themselves to matte finishes very well. Semi-gloss at a minimum.

@Joseph Corlett, is that because of scratches or fingerprints?

Lower grits expose more of the aluminum trihydroxide (55-60%) in Corian and other solid surfaces which appears as white. The white doesn't go until the abrasion gets finer. So a matte (solid) black Nocturne Corian appears kind of like a dark gray until the finish is finer. Scratches will appear whiteish.

Thank you! This is very helpful. I always appreciate how generous you are with your expertise!