Yep. Sunlight will cause the nicotine to darken in e-juice. This is oxidation of the nicotine. Oxidation is also caused by heating the nicotine. In that photo you will see that the unflavored 15mg juice I have in the carto tank is a little yellow. That's not from the Bank's Roses. This is because I have been vaping the same carto for a week and yellow nicotine oxide (cotinine) is starting to accumulate in the tank. When I refill the tank the juice I'm putting in is clear. By the time I've vaped the same carto for 10 days the juice is pretty yellow and strong tasting. That's when I replace the carto and clean the tank.
It would interesting to use sunlight for a quick steeping. I am sure you would have to monitor it since too long would kill the nicotine.
