The first glimpse of hairstyle “highlights” in CAS, was noticed in the Create-a-Sim gameplay trailer. I was able to check out the newest version of CAS in more detail today, and it was confirmed by The Sims 4 team that the ability to add “highlights” is only possible with certain hairstyles.
This means that you will not be able to customize any hairstyle you choose, but they claim to be working on adding a good amount of these “special” hairstyles for the base game. Additionally, you can change the colors of highlights, but only to colors available in the panel up top. Check out some screens and a short video of highlighted hairstyles.