SWIM really doesn't think that the yellow is necessarily oxidized DMT. The structure of a crystal will change it's color. A slow crystallization of DMT fumarate can result in clear crystals, but if the crystals are made more dense or of a less uniform structure, they will take on a darker hue (yellow to dark orange). Melted DMT will not be clear or white, it will be yellow, so if it doesn't crystallize properly (soft or oily crystals), it will likely take on a yellow appearance. It is true that impurities can disrupt the crystallization process, but SWIM doubts that coloration itself is necessarily the color of the impurities, apart from possibly some amount of actives like DMT n-oxide (unless using an incredibly dirty extraction process).
With acetone evaps, DMT will form a web of crystalline appearance, but when scraped up, it will be more yellow and even tan as it gets denser; this malleability is likely due to slight moisture contamination (possibly some acetone, but more likely moisture absorbed by the acetone and trapped in the crystals). The white appearance of some freeze precips and heptane evaps, merely depends on the size of the shards: smaller shards giving a white appearance, larger ones being clear, but both are essentially clear.
Regardless of whether they're yellow or not, they'll still be just as active.
