On episode 18 of the Loopcast, Sina and his guest discuss fashion and national security. Around 23:00 comes the money quote: "There's a lot of black hair dye involved being a dictator." Here's the logic:
[I]n a democracy, your hair turns grey very quickly in a four year term.... But in a dictatorship, the hair gets oddly darker: it just turns to an eerie shade of black.... [Dictators] want to remain relevant. They didn't want to get old.... They didn't want it to seem like they had been getting old and getting crushed by the responsibility of their job.
While a random sample of hair shades and a thorough hypothesis test is beyond the scope of this post, I'll let the readers judge for themselves based on the photos below. Note that one source of bias may be that US presidents try to look young and vital for the election but let their hair go after that.
(More US president before/after photos here.)