OK...I did some research and found this regarding a post on an E cig forum (Dont know which one as it wasnt in html format)... By someone called Nick Oteen who owns Decadent Vapours.
'Title: Re: Liquid changing color
Post by: Nick OTeen on December 06, 2010, 05:31:58 PM
Freebase nicotine is a very reactive chemical, and it will tend to change colour in solution (which colours it turns depends on what it's mixed with, though pink is typical in PG, and yellow in PEG.) Different flavouring compounds can also contribute to colour changes, though the main agents of reaction are light, heat and exposure to free oxygen.
It's a purely cosmetic factor IMO - I don't believe much (if any) nicotine is delivered as freebase in vapour anyway, since it will react immediately in the presence of the heat generated by a glowing filament (quicker than you can get it into your lungs.) This is probably why we don't get as much of a head rush from vaping as from analogs (analogs do a pretty good job of preserving the freebase, because they use ammonia to crack it, and it's inhaled in micro-globules of tar in a stream of oxygen-depleted air, that's already passed through the combustion zone at the tip of the cig. In comparison, you're sucking fresh, fully-oxygenated reactive air into your ecig, to mix with very hot, vaporized nicotine.) It would be hard to design a better way to degrade freebase nicotine if you tried, but we know from experience that vaped nicotine is still satisfying
For best storage conditions, keep it cold, keep it dark, and keep it airtight. But don't worry if it darkens - you'll still get the same nic buzz, and the flavour isn't unduly affected. I have some nearly year-old old cinnamint on my desk (deliberately exposed to ambient light and heat in a bottle half-full of air,) that's now black, but still tastes fine and delivers the same nicotine 'hit'. A similar vintage of menthol is still only very slightly pink, while vanilla is bright red, and rhubarb crumble is deep brown.'