Hair question

I recently had my hair dyed a very dark brown....I am wanting to go 1 or 2 shades lighter to make it a medium brown. My hair stylist recommended a shampoo cocktail to remove the dye. My first question is: if I had highlights underneath the dyed hair will those show through once she does this shampoo? and second, will it only lighten back to my natural color(it is quite similar to the color I had it dyed) or is it possible to go even lighter than my natural with this wash?