Highlights are changing color

Hello all! I have naturally dark brown hair that is colored a medium warm brown with golden blonde highlights. My problem is that the highlights are getting significantly lighter each time I wash my hair. I use salon brand shampoos and condtioners, specifically intended for color-treated hair, which I was told by my colorist, would minimize fading. Apparently, that's not the case with my hair! Presently, I'm using the Redken color extend line. Because of the fact that the highlights have faded so significanty, I'm thinking of switching to an all-over, solid shade in the brown family. My only concern is that browns can sometimes look boring or mousy, which is certainly NOT the look I'm going for. Anyone have any thoughts or advice?

Thanks!