Ever since I've been dying my hair blonde, I've been told by many people to treat my hair with a "purple/silver" toning shampoo or conditioner every few days to help prevent my hair from getting a brassy tone to it. Supposedly, this works by 'color theory' in the way that the purple toner in the shampoo is able to cancel out the brassy yellow colors in the hair. An example of such a product is the AG Sterling Silver line, one that I've been using myself (although I'm still skeptical). My question is, does these types of products actually work to keep brassy colored hair at bay?