A bluer light will make your plants greener. They think they're seeing the sky, and so they turn bright green to absorb all that direct sunlight they're anticipating. I have found this to be the case with 6500K fluorescent lights too though. For a while I was growing a few plants outside and a few plants inside under 6500K fluorescent lights. The indoor ones were much greener. I've never used metal halide lights for growing, but I imagine they're quite blue too.

The problem isn't that the bulb isn't giving off the right spectrum. It probably is. It's just that for the energy you're using and the heat that you'll be giving off, it's not worth it. These types of bulbs aren't sold in stores anymore because small fluorescent lights are so much better.