+ * ChangeListener handler for when a colour is picked in the colour chooser.
+ * The action is to apply the colour to all selected buttons as their
+ * background colour. Foreground colour (text) is set to a lighter shade in
+ * order to highlight which buttons are selected. If 'Lower Case Colour' is
+ * active, then the colour is applied to all lower case buttons (as well as
+ * the Lower Case Colour button itself).