looking for advice - my wife has her dyed red at a salon (she's natural brown, with a lot of grey) and is trying to go redder. the stylist recommended she use a clarifying shampoo a week prior to her next coloring and the stylist also said she'd up the developer to 20% from 10%. my understanding is the shampoo will remove old product but also some of the previous color to better prepare for the next coloring. then the 20% developer will allow her to lift the color more - ie make it redder. is all this correct? also - any recommendation on which shampoo to get to best prepare her hair to go redder? thanks!