Very interesting infoTony
Those colors are just interference colors that result from development of oxides formed during heating. As the oxides become a little thicker the colors change. It's all a matter of the wavelength of light and the oxide thickness. You are talking about Angstrom thicknesses. As long as you are seeing colors like that the oxides are totally adherent and you don't need to worry about even the slightest possibility that they will get into the vapor stream. Lots of times interference colors are developed either by heating or by anodizing to create jewelry.
I was researching why my titanium wouldn't form immediately that thick titanium dioxide like I've seen in other pictures and I came across a board on the dark side of the net where people use titanium nails for some recreational uses and they actually were commenting that grade 2 nails would quickly become dull and coated while grade 1 wouldn't so easily because of the oxygen content being much higher in grade 2.
I wonder how many people have actually grade 2 instead of grade 1. Most of this wire comes from jewelry businesses and I doubt there is as tight control as we might wish?
Did you start experimenting already with titanium?
Cheers
Tony