M.J.
A Pastor at my church as two kids...boy and a girl. Both look like punk rockers including black and white striped clothes with hot pink and chains. Both dye their hair every other month and wear makeup. Both outstanding and very full charactered kids. Their dad just loves on them and supports them in their choise of expression at the same time being firm if they were to step out of line.
Although it's important that your son have some ability to show some independance in a healthy way...like green hair gel, it's more important that the both of you stand as one. He is testing his independance in this small way now but if he knows you won't agree with his dad, he will eventually use this to his advantage because that's just what kids do.
When I worked for the youth department, I saw the clean trimmed kids being nasty to new kids while the punk rock kids were kind and inviting. They didn't learn to be nice by the way they dressed, they learned it by the example their parents set on them. Build the inside, stand as one, agree on a compromise because although I don't see the big deal in his hair being tinted, you both should meet in the middle. Maybe if he's good in class for a month, he can have that way. Something along that lines so it's not just a request and get type of thing. Make him earn the right to booger colored hair. That may suit both your points of view...good kid = freedom of expression.