Expression controls are really useful when working with color. Say you have many text layers in your composition and you're working with a client who keeps changing his mind about text color. (Sound familiar?). Every time he says, "Can you make the text white?" you have to change each text layer's color to white, one-by-one.
Then, when he says, "I changed my mind; make them green," you have to change them all again. To save pulling out your hair or his, try adding a Null layer and naming it Controller. Add the Angle Control effect (in the Expression Controls category) to Controller. Rename the effect Color Picker. Finally, add an expression to each Text Color property, always Pickwhipping the Color Picker Angle Control on in the Effect Controls palette. Now, when you adjust the Color Picker Angle Control, all the text colors will change at once to a new color.