If I understood the question, next to the Dimension or Expression on the Dimensions and Expressions tabs will be little + signs. If you click on them, it brings up options for controlling display of the Dimension or Expression. One of the options is Text Color. Click on it. It will open up a "Definition" box. This is where you enter an expression that returns a color. For example:
if(Fruit='Apples',red(),rgb(100,150,200))
