The NoDaks (at least the aluminum) are black, and the M4 is grey.
One good thing about the color, if it's off a little, it probably won't be as noticeable as two different color tubes being mated together. At least the entire tube will be the same color. I've never seen the NoDak extension in person -- is it too dark, too light, or just a totally different shade?
I'm assuming that the factory mag tube is held in place with some type of Loc-Tite substance, and that an application of heat will allow it to be loosened.
Anyone here ever remove their factory mag extension?