Ok, SWIM has yellow crystals that have worked well. He wonders if over time that yellow part might be bad for him vs pure white crystals. He uses STB tek, and had no burning sensation when smoking, so believes he got all the NaOH out.

Is it possible the yellow is leftover unevapped Naptha? Or is it an oxide? SWIM doesn't want to mess up his lungs over time. But he's noticed the stuff is fairly easy to smoke. Does produce some coughing (which he holds in) but it's very smooth.

Yellow stuff is most usually dmt-N-oxide, less so often some plant oils, depending on the technique of extraction. Freeze precipitation should not theoretically give much oils, so in any freeze precipitated-derived spice the yellow coloration is dmt's oxide. If the naphtha pulls were evaporated then some plant oils will have made their way into the final product as well.

dmt-N-oxide is fine, there is nothing wrong with that and the experience is no worse at all from smoking pure white spice. Maybe a little bit gentler but that's perfectly fine. SWIM's FOAF never really understood why people want the yellow out. It's nice practice learning how to recrystallise and get pure white dmt but a bit pointless from the point of the experience seeking.

Yellow spice is roughly estimated to be composed of ~85%-95% pure white spice
and the rest 5%-15% of it is dmt-N-oxide. Pure dmt-N-oxide is a viscous oil (not solid) with deep orange colour. One can easily see how a tiny bit of it mixed with pure white spice can give yellow spice.

Hope that helps!

Would smoking pure DMT oxide have any effect?

Beyond the shadow of doubt. People report a slightly gentler experience, but a dmt experience nevertheless.