The 15s and 16s both had titanium bodies which (as I recall at least) don't take on colour as well when they're anodized, so that could be the cause of drab colour ways.
edit: It was only the Pros and up which had titanium bodies. The 17s are all aluminum.
Anodizing titanium creates an oxide layer, the thickness of which varies with the voltage used. The thickness of that oxide layer determines which wavelengths of light it refracts [0]. In practical terms, your choices for color are pretty limited[1].
I'm not a chemist, but I looked into this years back when I was wondering why everything titanium is offered in the same couple of colors. Personally, I like the plain gray.
edit: It was only the Pros and up which had titanium bodies. The 17s are all aluminum.