There is a misconception that you should leave your hair dirty before getting highlights or hair color. Dirty hair will most likely not affect the hair color (as long as there isn't a ton of build-up), but generally, you want clean hair before getting color or highlights. If you think about it, dirty hair coats your cuticle in more junk that the hair color has to break through to work properly. We want to make it as easy as possible for the color to take.