People actually see colors slightly differently, and possibly have different thresholds for what they think different colors' ranges are too.
I'm quite certain that they see colors slightly differently, because I myself have slightly different receptors between left and right eyes. Both test as being fully capable (not color blind at all) and yet one sees reds more strongly, and the other blues. It's almost impossible to tell normally, but if I look at, say a very grey road, one eye will be able to see the individual red pebbles fairly easily, and the other eye will just see blue/grey primarily.
As to purple/black stone/rock... they are purple in your inventory, but more like inky black when you actually place them. Very, very, very dark blue/black color with a bit of grey mixed in.