If it turns out a shade too dark? Wash it with hot water and a clarifying shampoo a few times. It will fade. If it’s too light, you look like a confused skunk. (Personal experience, again.) Just buy a darker brown demi-permanent and glaze over it. You’re not a failure. You’re an artist who made a happy accident.
Also, please wear gloves. I did not. My hands looked like I had assaulted a chocolate bar. Very suspicious in public.