Create a painting app, something that applies colors or paint or texture as the user moves the mouse or some other interaction.
Inspired by scratch painting, my painting app shows black at first. But there is actually a colorful layer behind. When you scratch the surface, the color will be revealed.
The random color is generated with 2D perlin noise. I also added a change-brush-size-by-mouse-wheel feature. You can see my code here.