The entire string can have only one color. It's not possible to give some stars one color and other stars in the same string another color. You can change the color by clicking on the + character in front of the dimension on the Dimensions tab, then click on Text Color and enter an expression in the Definition input box that calculates a color. Examples are color(1), lightred(), rgb(0,255,255)
Oops. My mistake. In Qlik Sense you can change the color of the bars, but not of the dimension labels. If you need that kind of functionality you'll have to find or create an extension or widget that offers that option.