You can't copyright a game mechanic, but you can copyright content. This is why Words With Friends has a different board layout, tile distribution, and tile point values from Scrabble.
In that Set C&D, the infringing implementation is using art assets from the copyrighted Set game. Presumably, a version with different colors/shapes/etc (and a different name, so as to also avoid trademark infringement) would be totally fine.
In that Set C&D, the infringing implementation is using art assets from the copyrighted Set game. Presumably, a version with different colors/shapes/etc (and a different name, so as to also avoid trademark infringement) would be totally fine.