I'm %99 sure it's the nicotine. Try mixing a small batch with zero nic (add nicotine a few days later), and I'll bet it never changes color without the nicotine (as long as the flavoring is clear).
Side note, I think colors of flavors, like you buy mango flavor concentrate and it's orange, or strawberry and it's red, is not a good thing. It's just another thing in the flavoring that is unneeded. I think the flavoring is the least safe part of the mix. I think it's added during manufacturing. Because we are using universal flavors that are meant for candy making etc (eating), not inhaling the flavors which can be dangerous but not as dangerous if eaten. Most flavor vendors state "we do not add coloring to any flavor" but what I think it is that these flavors are originally powders (and powdered coloring is added), and then a big vendor buys these powders (not made for vaping) and they just add PG or ethyl alcohol or VG base. So technically "we don't add color" but the powder did have color. Possibly so.
This goes for standard artificial flavors and natural flavors. CO2 flavors (very pricey and I wasn't crazy about them [Medicine Flower]) most likely don't have color added and possibly strawberry CO2 would be red. CO2 usually if not always means that it was actually extracted from the flavor it says on the bottle (banana, strawberry, tobacco, etc) whereas artificial are man-made petroleum bi products (mind blowing how they can replicate flavors from molecules like that). And "natural flavors" aren't from a banana or strawberry but they are from natural sources or flavor molecules that just so happen to taste like banana/ mango etc when they're all mixed together which can be comprised of a whole slew of different natural compounds derived from various plant sources and plant roots etc etc. If the sources are Organically grown, then you have Organic Natural Flavor.
Despite all that, artificial flavors are actually often safer than "natural ones", but possibly not as often safer than CO2 extracted ones. And Artificial flavors often don't have as much environmental impact as Natural ones.
essential oil flavors is another thing, those a usually steam distilled or solvent extracted from the actual flavor the bottle says, but a lot of those aren't safe for vaping (a well-known vendor selling essentials for vaping though I would those flavors are tried and true, safe as can be).