I think that some people actually prefer the yellowish stuff. Or tan, or, any other color it may turn. (almost any color). I have some yellow, light yellow, and tan, oily goop, that eventually hardened as well. Once my purdy white goodness is all gone, I'm thinking about trying some of it out. See what happens.
But, if you don't want it to be colored at all, a re-x would be the best bet. You'd want to heat up your solvent, in my case, it would be naptha, using the boiling water technique. Open flames, and naptha.. no good.
And, most of the stuff in your off colored dmt is most likely just some oils from the plant material itself. Some other alkaloids maybe, or even just plant fats. From everything I've seen, it's actually not harmful to you at all.
But, to each his own.
Re-xing would definitly be your best bet though, I think.