It's going to sound cliche, but it's the insides that counts.When a girl changes her hair color,it's kind of an ego boost.and If you like her,you like her,you shouldn't be able to help it.Good luck : ]
If it still looks good then I don't see why it should be the reason to not like someone. Its just hair color, if a guy stops liking you because of your hair color, he's just shallow and not worth any time.
But, I don't think it would matter. Then again, I am no guy, the guy could have his preference.