Ok, it's a custard right?? A really sweet flavor? You've got a combination of things going on to produce a harmless result that at the worst will just give you a bad taste in your juice if you let it sit too long...
First of all, as said above...Nicotine oxidizes over time with exposure to light and even more with exposure to oxygen. There was a study to see how long it would take the nicotine to degrade and loose it's potency a couple years ago if left out open and exposed and in light (both natural and artificial,) and the result of the study is that despite the change in color from the oxidization process that it took almost three months to cause a loss in strength in the nicotine and even then it was minor, it was like a 1 or 2 mg difference. This can be easily and quickly proven with a still. You can take perfectly clear nic base, heat it up into vapor and let it travel through the still and re liquify and although the liquid was a nice amber color it was the same potency and just as pure, the only difference was the change in color from the oxidization resulting from the process of being heated to a vapor and turning back into a liquid again.
Also, along with this and espeically with sweeter liquids or more sugary flavors they can change colors more drastically as well...when you fire a vape up not all liquid that is in close proximity to the coil is instantly vaporized. some of it is super heated close to the vapor point but doesn't quite make it and there is a small amount of vapor that doesn't make it and is returned to it's liquid form causing the same process as described above to take place inside your tank....that's why as the day goes on and your tank get's lower the color can get darker....it's a result of some of your liquid getting "burned." This can change the flavor of it slightly but it doesn't make it harmful...just icky tasting at worst...and it can happen just as easily with a fresh coil as with an old one...just with an old one you have a bunch of gunk build up from this process that helps make the flavor even more....well...umm....interesting for lack of a better word will do.
Hope this helps...happy vaping!