A random walk of particles over a fixed canvas, with an hidden image mask influencing the behavior/appearance or the particles. The color scheme is chosen randomly from the 20 most popular palettes on Kuler. To run this you'll need the following:
You can watch a video for this here: https://www.youtube.com/watch?v=JjVtBkHGwFg
While the application is running, pressing space restarts the painting with new colors, pressing 'r' starts/stops recording and 'c' captures a snapshot.
There's also a simpler version using processing.js available in the pjs branch.