The model currently only has one field: red-value.
- Add two more fields: a green-value and a blue-value.
- Add two more setter methods: set-green-value! and set-blue-value!
- Add two more getter methods: get-green-value and get-blue-value
Remember, the model is just the data. Therefore, it should only be a collection of fields and methods to modify/retrieve the value of those fields. Nothing relating to a GUI should end up in the model.
Don't forget: when you change a value in the model, you need to send an update message to everyone who is an observer! (Have you figured out how that is done yet? It's there in the code, and in particular, defined in the parent class of the model...)
You should still be able to run your code at this point. Even though you've changed the model, nothing should have changed in the GUI or controller; therefore, it will be the same, one-color-swatch program.