It doesn't seem to take much to discolor DMT (or for that matter many other substances) often a very small amount of impurity in some things can leave them looking ugly as hell.
Though yellow DMT crystals are far from that. I've had some of the citrate salt in a stoppered 10ml microscale flask for storage, in a fridge for months at a time, at one point, and noted no discoloration.
Tryptamine freebases (what you want for vaporising in the case of DMT, although at least 5-MeO-DALT salts of some variety appear suitable for vaporising, not sure what the salt was though)