Re: Thread coloring
In the absence of solvent or mechanical abrasion which will remove the pigment, the marker color should remain. That has been my experience.
The source of the odor is the solvent in the marker. As the solvent evaporates, the odor diminishes.
If your goal is to alter the head color, you only need to swipe an inch or two of thread with maker prior to completing the head. The amount of marker pigment/solvent in such a small application is trivial.