Yes it is, Certain components in flavouring will react with nicotine over time.
I think this statement is
close but not completely accurate. Reactions with nicotine would be a concern, since now we are vaping different compounds, which we know nothing about. Of course, not wanting something to happen does not preclude it from happening, but I think the only
reaction here is nic with O2 to form N-oxides, which are colored. These oxides can, however, pair up with flavor compounds, since N-oxides have charged atoms. This pairing up does not at all mean they react, but the colors of such charges compounds are strongly associated with their environment, including any flavors that pair up with them. I've seen this pink coloration happen with all my vanilla-based e-juices (Fr. Vanilla, Bavarian Creme, etc) so I am guessing the vanalins are pairing with the N-oxides. I've even seen it with unflavored 8 mg juice, which tells me the N-oxides are pairing with each other.
The point is I don't think there are any further reactions, or I hope not anyway, but color turning is not something to worry about. Color turning with the creation of foul taste/smell, however,
is of concern, as this means your flavor has decomposed, that is reacted to fall apart into smaller compounds that stink...fruit esters will do this after a while, as in months. Bad tastes or smells mean toss them...they will not get better, and you cannot mask the bad smell/taste.
We cannot stop nic from forming oxides with O2, and it takes only trace amounts to color a juice. The oxides are harmless, but can form strong friendships with other compounds present, and this will can their color. Much like a couple that marries will not become conjoined people, but the relationship can make for some colorful interactions!