I'm sorry I have to say this, and you'll probably curse either me or the game, but it's TOTALLY RANDOM.
I have played through the game four times or so, I remember getting a bronze one at least three times. And I'm sorry, but there's no place you can possibly get many color crystals. Below is kinda spolierish (for you at least), it's about how bronze crystal looks like (check at your own risk)
It's VERY similar to orange (I can't tell the difference) and actually looks like red with a bit of yellow in it. It's also brighter, so the volume of the light beam is bigger (which is why I prefer bronze/orange over red, they look better in combat).
Walk among them... these beasts that are less than you are. See their strength, see how easily you fall to their muscle and skill. Why do you stand for this, why do you submit to the flesh when death is bred in your bones?