Is it lightning? Because of his specialty. Flying *THUNDER* god technique
Or is that just yin or yang or yin\yang chakra. Because its space time jutsu. I dunno?
I think he has YIN, YANG, YIN-YANG because of FTG. If you try to analyze the affinity FTG is made off, then you will come up with the 6th element Yin and Yang. He could also have earth element because of the jutsu that can tell the location of the enemy by touching the ground. Ground= Earth. Another possible affinity is wind. He tried to stab A using a kunai even though A is using lightning shroud. Only wind is stronger than lightning. Minato was considered by Jiraiya to be a shrewd person, and was not a man who would do something without reason so it doesn't make sense why he attempted to stab A with just a kunai if he knows that he can't hurt him.
Minato's affinity is still a mystery but there were info given so all we have to do is gather those info then analyze. I agree with you that he may have YIN/YANG chakra because of his FTG.