I looked at the code of either this thing or something similar in the past (don't remember). Basically there is a "swarm of particles" that follows your mouse w

I looked at the code of either this thing or something similar in the past (don't remember). Basically there is a "swarm of particles" that follows your mouse when you draw, and they orbit around it with some noise added, as if it was gravitation simulation. The path of colour trails each particle and it may or may not change over time. So if you stay in one place for long, you get more colour and if you do a quick stroke, particles follow quickly and trail becomes thin. You can also add mirror or star symmetry.