Definitely an interesting topic on color change. We did a bit of an experiment with some 10ml bottles to see what environment caused the most tanning of a liquid and in our simple test, light was the biggest factor.
VG content also seems to affect the 'pinkness' of a liquid as when we mix pure nic (clear to slightly tan) with VG (clear), we usually end up with a pinkish base. We don't get this with an all PG mix.
Check out towards the bottom of the newsletter for some pictures of our tanning test: https://www.madvapes.com/Newsletter-81911_b_5.html