This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I'm not sure what your DrawingWindow is, but I'm going to guess it is an awt Component or Swing JComponent of some kind. In either event, look at the method addMouseListener(). That will lead you to look at MouseListener. When these are all linked up, a mouse click on the component will fire the mouseClicked() method and you can change color there.
Give this a try using the javadoc and maybe some Swing tutorials on the web. If you get some working code or "almost working" code, bring it back and let us see how you're doing!
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi