Yea over time if you're reusing the same solvent it can turn more and more yellow. And yellow DMT in a solution of fresh solvent can turn it yellow too.
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I think you can clean the solvent up by washing the solvent with an acidic water solution. An acid wash should convert and separate any DMT left in the solvent (you can't practically freeze out 100% of it).
Another thing to try is an activated charcoal wash. I think the idea is solutes, including DMT, bind with the AC.
It can be a pain to filter that so distillation might be the best thing after an AC wash.