I think there's opportunity for more expression if you depict negative values as a distinct state. For instance you could depict negative values as a 180 degree flip the hue (just a suggestion lots of options there)

When I made my (more esoteric) thing https://c50.fingswotidun.com/ I took a long time to consider the features before I froze them to allow people to submit their own code. I think you might have more scope for expandability given you already have modes of operation radio buttons for shapes.