I know you have already established how much the RAM are covered by the NH-D15, but I thought I'd share a
picture of my actual setup with a NH-D15 on a standard ATX MB and Corsair Vengeance LPX RAM with a height of 31mm.
As you can probably tell, it is completely impossible to actually see the RAM because of the cooler. (There's plenty of room for taller RAM though, if you have the room to move up the front fan a little bit. The fins are cut away quite far up the front cooler fin stack, so they won't get in the way).
So if you end up using an NH-D15, I'd say RGB RAM would be a waste of money, unless you find a cheaper RGBkit with same specs as a non-RGB kit of same performance.