The sharpie will wash out and be totally gone in about 2-3 washings.
I just used my black pigment pen. I don't even know where to find a brown one without mail ordering, and I'm not about to go through all that hassle just to sign "Bonnie MS" on some quilt blocks. My sig was so tiny I didn't figure it would matter. I'm sure they will be getting plenty of sigs in black.