I think you are correct about the nicotine.None of the speed steeping methods I've tried have had stellar results to say that's what is going on.
Except seed steeping. Having a little of the old aged recipe does seem to transfer to the new mix faster.
Even TFA Strawberry. That's the only one I know that goes from colorless to bright yellow. I can't say that has anything to do with aging for flavor since it doesn't need aging. But if it doesn't happen with 0mg I can say there's some interaction with flavoring chemicals and nicotine since my strawberry has been around for years without turning and my nicotine bottles sit out as long as the strawberry mix takes to turn yellow.
Nicotine starts the process of blending the flavors together(steeping,aging). And i also think a little heat helps it also(altho I usually do not use heat). All of the other ingredients we use do not change in and of their selves. So it's the process of elimination since its the only chemical seen to change even when not interacting with other chemicals.