Garage recolor won't work?

This is probably caused by the fact that the functional mural object is meant to be painted on -- which means that any texture you give it might just get graffitied over during gameplay anyway. If that's something you specifically want, perhaps someone will have an answer for you.

On the other hand, if you'd rather Sims not paint on your recolors, you could recolor an existing deco-only version or make your own. (In the latter case, the first thing I'd check for is whether there's a deco only version in debug.)