Hilo Bottle Finder said:

Have you guys ever had a straw bottle start turning amethyst. I've had the Arctic in the son a few weeks and seems like there is a slight tinge of amethyst. Now I am afraid to subject the bottle to ultra violet rays.
HBF> When a bottle has selenium in it, it will have a yellowish color to it,
it will not get any darker if exposed to UV lights, yellowish color bottles does not get any darker, what you see is what you get...
On the other hand manganese will turn amethyst, the more manganese in the glass the darker it will get, some of bottles has a mix of both, I also had a straw/smoke/amethyst Register Arctic that I sold in past so I know what you are talking about.
As for my smoke straw Hollister I keep it out of direct sunlight to keep it yellow, I don't want it to turn amethyst in case they are traces of manganese in it.
I'm not 100% sure but I've noticed that when a bottle has smoke and straw in it tends to turn a light tinge of amethyst if exposed to UV lights, I think it's something to do with the smoke, it probably has some traces of manganese in it and when UV light hits it, the manganese is activated and can over ride the selenium, causing to lose the yellow color which is replace by amethyst. It seams as though your bottle has manganese in it,
if you do not want it to turn any darker amethyst you should not expose it to UV lights...
Remember it all depends on how much manganese is in the glass and how long it is exposed to UV lights will determine on how dark amethyst the bottle can get... I've seen a smoke/straw bottle with a tinge of amethyst turn darker amethyst and lose all the straw color when the bottle was exposed in a germicidal light box, another way of exposing a bottle to turn amethyst faster and darker, powerful UV rays exposing the bottle 24-7 will speed up the process in turning the bottle amethyst, a process that can take many years with the sun, but can be done in just a few months...
Not to be confused with irradiated or "Nuked" bottles, still another process to turn a aqua or clear bottle dark amethyst purple, smoke or amber...there's a few nice amethyst purple Hawaiian bottles floating around and a lot of colored Hobble Skirt Coca Cola bottles, those amethyst, amber and smoke colored Cokes are all irritated, some collectors like it some don't and some are totally against it...They are nice but...Got be careful and not get mislead thinking it's a rare bottle...
Maybe the other guys might have more to say about this... Here is couple of pictures of a straw colored bottle that basically has no smoke in it and irradiated bottles that once was aqua in color>>> I hope all of this helps.
Aloha 808Kaz