Grandpa is making me re-dye my hair?

I dyed my hair black from dark brown today. My grandparents saw it and didn't say anything, but later my grandpa came over our house to give my mom something and said I have two weeks to re-dye my hair or he is selling my car that he loaned me some money for (the title is still in his name). I'm 18, have a 3.5 GPA, a 20+ hr./week job, and I'd like to be able to make my own choices, even small ones like what color my hair should be. At this point, I'm pretty much stuck with my hair since it's black, and even before I dyed it I had plans to get it cut the next time I get the chance so it'll look What should I say to him?