The meaning of form is made clearer by its opposites: The contrasts. We would not recognize day as day if night did not exist. The eye accepts a line as long, when a shorter line is present, and the same line is taken as short, when the line compared with it is longer.
In the same way, colors are intensified or weakened by contrast. Seven major kinds of color contrasts give colors their visual character, and constitute the fundamental resource of color design.
This is what this app is about: "Color Creation" and "The Seven Color Contrasts".