How do I change my vocal timbre?

This is a mystery to me as well, but it CAN be done, as there are a lot of comedian/impressionist types who can imitate a lot of different singers. But short of that, you can affect tone my raising your soft palate, by lowering your larynx (or in some cases raising it, as in some old-timey bluegrass singing), by directing your tone toward various postions on the hard and soft palate, and by manipulating jaw, mouth shape, tongue position, etc. A lot of these tactics are frowned upon by vocal instructors, as they violate proper technique, but for expressive purposes, especially for spicing up a sound here and there, you use what tools we have.

What is the problem with your tone, and what do you want to sound like? It's easy enough to judge one's own pitch, but when it comes to tone many of us cringe when we hear ourselves, which is not the reactions of others. Recording yourself regularly is crucial, as it will enable you to experiment with your tone (try out mouth shapes, etc.) and acclimate yourself to the natural tone of your voice, which I'm willing to be it a lot better than you think it is.

I like to sing a lot of classic rock stuff, but I think as far as voice is concerned I always wanted that powerful, soulful, gospel kind of voice. Like when your singing high notes they don't sound light but heavy. Like Chris Cornell, Eddie Vedder, Robert Plant, Roger Daultrey, Freddie Mercury, etc. They all can hit high notes, aside from vedder since he's baritone, but they have a voice that's loud and heavy, like even though they're 20 something they sound like they're 30 or 40 something. I just want a more brash and layered voice, because right now I kind of sound like young voice and an old voice mixed together. It's very strange.

I think the traditional method for getting that kind of voice is plenty of hard drinking and cigarettes :)