You can spray your hair with hairspray, use pastels or chalk on the tips, then use more hairspray on top of that. It will wash out as soon as you rinse your hair. Then show them how much you really like it any convince them you wont go overboard. :)
You can convince your parents to let you dye your hair with Kool aid by letting them understand the benefits that dyeing your hair would bring. At times, it may be as simple as improving your self esteem.