This iExpression links a color to a slider (or to some other 1D property - like audio keyframes). You specify which value the color should have for the minimum and maximum slider value, respectively and the expression then blends between those colors as needed.

By linking many different color properties to the same slider, you can control a lot of difference color changes with a single slider.

Example

In this example, we linked the colors of the hair, skin, eyes, eyebrows, nose and mouth all to the same slider to control all of them with a single slider.