the webassist extensions do not have the capabilities of doing what you are trying to accomplish.
If you are not going to create an image for each color combination, you would need to use PHP or javascript to create the image dynamically, Not an easy programing task.
In PHP you can use the GD library to generate an image:
book.image.php
I think you would need to use a combination of AJAX and PHP scripting tied to the onchange event of the radio buttons to change the image.
I am not aware of any extension that will help accomplish this. It would require extensive hand coding.