There's either the whole point of it being a manga and it could be just cause he likes orange hair. or it could a play on name like they do when the characters make reference to his hair, or it could be because of the recessive trait theory. I never seen Masaki's actual hair color as I don't always trust the anime with actual character colors (they had it as orange in the anime).
It could be similar to Baki from Grappler Baki. His hair was orange and then it changed to dark red in less than five years even though his parents have brown and red hair, he was born with orange hair.