Is this a stupid idea ?

I really want to die my hair but my mom wont let me. Its like milky chocolate brown colour and i wanna die it really dark brown so i was thinkin .. see im going to my cousins for a week and i wont see my mom so if i buy hair die there and then die my hair will she notice because she hasnt seen me in a week. & will it be very obvious when the roots start showin ? Thanks (: