So if Ken's blonde hair is dyed in the canon setting
Then he spends of time dying his hair....... lol.. I am pretty sure he will have to dye his hair like at least once every month so it doesn't look weird (with the root portion of hair growing out as dark or dark brown)...
Anyway, I just want to talk a bit about the casting on Ken and Gouki and show my reasons of supporting the cast in this movie
Even though Ken's ethnicity is 3/4 Japanese, I feel they actually did a good cast on Ken in this movie because 1) having played SF series since 7 (yes, I started to play sf2 when I was 7), Ken has always appeared like a white guy to me, and I think MOST old-time players like me had always thought he's actually white, despite the fact that the later published official guide stating he 's only 1/4 Caucasians and 2) it's totally ok for a 3/4 Japanese guy looking like white guy because it is possible in real life when you look at people like Hirai Ken, a Japanese pop singer, both of whose parents are considered pure Japanese ethnicity but Ken himself who looks totally like a Caucasian or a mix (could be possible that one of the ancestor being foreign ethnic and passed down the gene recessively and finally started to show in Ken's generation).
So yes, I am totally ok with the Ken in this movie and in fact I think it was a good cast. Just the kind of Ken I have always had in mind. And the ethnicity is not problem due to my reasons above.
And then there is the issue of Gouki, whom was played by a African-mix actor. However, I also have no problem with that, because Gouki's appearance has always been illustrated as demon-like due to the influence of the Killing Hado. A demon-looking character, in my opinion, can be played by actor of any ethnicity as long as there is enough resemblance after the make-up, and in this case, I thought the Gouki in the movie was decent.