HOW TO WORK WITH COLORS
The color can be changed either for the whole book (global color assigment) or for the selected text fragment only (local color assignment). Being in the Reader mode you can change global color only,  while being in the Editor mode you can change both global color and local color as well. 
Changing colors from the Editor mode.  If nothing is selected in the main Book Designer window, the color is changed for the whole book. Otherwise the color is changed for the selected text fragment.
 
To change the foreground color. Check "foreground color" checking box and press a desired colored button.
To change the background color. Check "background color" checking box and press a desired colored button.
 
Comment. You can change the color as many times as you want without closing the "HTML Colors" window.