I think a lot of the old school corals would fit the bill. Porites is a great one, as you mentioned. Turbinaria reniformis is tan to yellow and has a really nice random plating form. The Leng Sy Monti cap has a purple rim, but is more muted than most caps overall, and can develop interesting vertical nodules. Candy canes can be found in brown with green centers or gray/blue colorations. Fox and bubble corals, Duncans, Duncanopsammia (formerly Turbinaria) peltata are more options. I always loved christmas tree worm rocks, and while the worms are colorful, the corals they inhabit are typically less so. Astreopora and Pachyseris are a couple of oddball corals that don't get too colorful.

