I haven't done much research on different brands of nicotine as actually we only have a few that are extracted here in the U.S.
The last I checked I recalled
three companies that were actually manufacturing nicotine, the largest being DOW, the other two I would have to look up.
It is entirely possible that heat or oxidization could cause some discoloration, but normally the chemical composition of nicotine tends to turn to a yellowish brown when exposed to the air, it may be also perhaps be something in the VG.