One thing you could do is do a Save As and modify your program so it has three color sliders and only one color swatch. Then, your sliders could control the red, green, and blue values of that swatch simultaneously.
Another possible extension would be to eliminate all of the repetition in your code: is there a better way to build the GUI and callbacks, so we don't have all of this copy-paste work taking place?
Lastly, you could explore how GUIs are put together more generally. Throughout this lab, I've linked to the PLT Scheme documentation for the GUI framework (called MrEd). Perhaps you might want to go on and explore the GUI framework further. The PLT Scheme GUI framework is built upon the open-source wxWindows framework, and runs equally well on Linux, Mac, and Windows. I have often created small applications that I can then compile, distribute, and run on all of these platforms using PLT Scheme.
For example, the simplest code I can write to put a GUI frame on the screen is as follows:
#lang scheme/gui
(define f (new frame%
(label "Uber")
(min-width 300)
(min-height 200)))
(send f show true)
This creates a new frame% object, and then I ask that object to show itself. Easy-peasy.
Or, you might go look at other object-oriented frameworks, and see how you would implement this application in your favorite language. The concept of model-view-controller should be the same: it is just a question of how much work is necessary to get everything tied together.