I have black (really dark brown) hair, I want gray. How?

I have a project on George Washington Carver next week and I want to look the part (obsession with bonus points). I have normal african american type hair and I've never dyed before. I want to know how to get from the regular African American black/extremely dark brown hair color to a gray, even dark gray. I don't want orange or green or red or any of the other "accidental" colors involved with self-dyeing. Please help.