I'm not a good one to answer, because I've never colored my hair. But I do have another suggestion. Have you considered having your hair frosted? That way when the roots start to show, it looks more natural. I have one friend who was dying her brown hair, switched to frosting, and no one noticed when she stopped having it frosted and just let it be natural. Grey or white hair can be really beautiful, and is not the age-related "stigma" it once was.