"No one has all one-tone skin. We would be completely flat and cartoon-like if we did," says Rez. We all have shadows, contours, freckles, and different undertones in various areas of the face. And just like we have balances of neutral, warm, and cool tones in our skin, we need them in our hair. "Our hair color needs to reflect that, especially if we are going lighter," Rez says, "so I always make sure there is a mirrored equal balance of those tones in my clients' hair color in order to be a flattering color change."
For example, Ciara in the photo above has overall cooler undertones than Zendaya, so Ciara's color ends up in the taupe region of the blonde spectrum, while Zendaya's is more golden. Their skin isn't 100% cool or warm, but that balance will look slightly different for everyone.
Achieving the ideal tonal balance is not something all colorists do automatically, but Rez accomplishes it using a customizable technique he invented called the Midlight color technique (sort of like a more complex balayage), which basically means he incorporates a client's base with multidimensional highlights that are tonally balanced and connected.