You are right, ammonia wash can help get rid of any residual lye if its presence in the product is suspected. Still, the only way lye can find its way in the NP solvent is if any parts of the aqueous solution found its way in the NP solvent.

Re-x definitely the way to go to separate the N-oxide from the dmt. Plus, if the yellow is from oils, they will be very soluble in naphtha and they are less likely to precipitate out (while dmt will).

If SWIY's re-x warm naphtha turns really piss yellow when mixed with the yellowish dmt, then the yellow is very likely to be plant oils. That is because N-oxide is insoluble (or very badly soluble) even in warm naphtha and one actually sees it falling on the bottom of the re-x container as a yellow oil, while the dissolved dmt (and suspected plant old) dissolve.

Hope that helps!