Creating generative art with Scratch is relatively simple: when the green flag is clicked, put the pen down, forever, turn 2 degrees, move ten steps, and if on edge bounce. Click on the green flag and the sprite will glide around the stage drawing nice circular arcs.
You can play around using "change pen color by 10" to create a rainbow pen and adjust line thickness with "change pen size by 1." If you want to experiment with the "stamp" block, use it along with the "change color effect by 25" and "change size by 10" blocks.
Programming an array of squares (what I'm calling "pixels") which change color when webcam motion is detected behind the sprite turned out to be easier than I expected. The code is simple: when the green flag is clicked, forever, set color effect to, video motion on this sprite. In order to align the squares precisely, I added the "go to x y" block, which unfortunately had to be set individually for each square.
After playing with the project, I wanted to increase the change in color. I multiplied the video motion so that the color would change more drastically. The amount of "video motion on this sprite" is calculated as a number between 0 and 100 (it sometimes will register -1 or -2). The color effect shifts the hue of the sprite. A color effect value 100 shifts the hue 180 degrees and a value 200 shifts it 360 degrees. Multiplying the video motion by 2 would allow for the full range of hues, but unfortunately the maximum amount of video motion (100) would result in no visible change in hue (360 degrees... back to the original color). In the end, I decided to use a variable as the multiplier so that I could easily adjust the amount.
You can click here to get to the project on the Scratch website. I have also embedded the project below. Scratch is currently Flash-based and will not work on iOS devices.