Those famous red walls of the Grand Canyon aren’t really red. They are several different colors (because they are several different rock layers), but the main one is limestone, whose natural color is pale grey. The walls only look red because red dirt from above has washed down over them and “painted” them. So go ahead–try to imagine the Grand Canyon as grey. It’s hard, right?
I know this from talking to a Grand Canyon guide. They really know their stuff.