"... set the corresponding menu items' IMAGE properties to the special value NULL.."
What are the "corresponding menus" ? the menus in the context menu and toolbar?
For example, suppose the user selects the Text Color menu item to open the color selection dialog, picks orange, and clicks OK on the dialog. Then the program should set the Text Color menu's Image property to null so that menu item doesn't display any color sample. (Because displaying an orange color sample hasn't been covered yet. Unfortunately general drawing like making a color sample of an arbitrary color isn't covered until lesson 39, I think.)
Similarly if the user picks a color for the Background Color menu item, set that item's Image property to null.