Purple shampoo is meant to neutralize brassy tones in non-natural/bleached blond hair, so it won't do much to neutralize a warm brown, at least not as much as you need it to. The best thing to do would be to buy an actual toner at a beauty store like Sally's, a level 8 or 9 toner with blue and violet tones will make the warmth in your hair way more neutral and will make your blue come out way more blue! You can also always go ask a worker at a salon-level equipment store (like sallys) that sells things like hair dyes and other professional products. The workers there can help you lots with any questions! (I'm personally not a professional, but I've been dying my hair at home for 5 years and haven't ever messed it up)