*SPOILER* Didn't you know his change of hair colour plays a major role in the film?? I heard that Simmons' henchment captured him at one point and tried to blonde his hair again. Leon actually gets weaker if he's blonde, and stronger when he has dark hair. That is how important noticing his hair colour is.
I made my gamefaqs account when I was 12 years old hence the username lol.
it still looks like dirty blonde, i should know my hair is the same color, it tends to look darker too if it goes longer uncleaned too so possibly just making him look like he's been going at it for a long time.