This is a really interesting card, but not adding mana makes this card good EXCLUSIVELY when you have the mana to use it. It not making mana is a huge detriment to its usability. That was probably the balancing payoffyhough, so it makes some sense. Maybe it could tap for colorless, but only can be used to cast artifact spells or to activate abilities of artifacts.

I think this solves the problem in an interesting way.