there's no need for you to change the way you look, he's with you, obviously he is attracted to you and if all it comes down to is hair colour... well it's pretty shallow.
My boyfriend drools every time he sees Eva Mendez, Jennifer Aniston, Beyonce... pretty much anyone lol and I am neither part peurto rican or whereever Eva is from, nor a perfectly tanned, blue eyed american gal, nor an african american chick from the south.
Basically what I am trying to say is, EVERY guy is attracted to some model / celebrity whom without the stylists, money or vigourous beauty routines would look pretty much like any other girl on the planet, lets face it they certainly don't wake up in the morning looking like that! It's a fantasy, and there is absolutely nothing you should feel pressured into being anything like. So what you have brown hair?..it's just a colour.
Let him have his fantasy and stop focusing on what colour hair you or he prefers. Chances are if you do try to change, it will look badly on you for trying to 'fit' with what you think his ideal woman is. Be yourself, love your hair colour and to be honest, if he hasn't said anything about it, then there isn't a problem. And if he did, so what? He met you with brown hair i assume, and he's still with you now!
Focus on your realtionship and not trival things such as this, I'm sure your boyfriend, like mine doesn't really care what us girls do with our appearance as long as we have confidence and make an effort to preen ourselves once in a while!