The Chromatic Collection is a set of textures that gives an extra layer of customization to the hairstyles.
You can mix colours, add ombre gradients, highlights and tint the roots, and all of that while keeping the files small to save disk space.
The Sims 2 Hairstyles
The Sims 3 Hairstyles
Remember that you must have The Chromatic Collection installed or the chromatic hairs won’t work in your game.