Explain to them candidly that you want to express your individuality as part of forming your own identity. If they have concerns that your haircolor will affect your ability to get a job, or a college interview, or it just violates their personal belief system, they still may not let you. This is a tough one, but your best bet is to speak to them from the heart with well-thought out reasons. And if you already know why they don't want you to dye it, try to address those concerns specifically when you speak to them. They may come around.