Are you fading out images in the HTML5 canvas by painting a transparent black rectangle over the top? Did you test your animation in different browsers? You may be unaware that ugly gray remnants are being left behind. I discuss the problem, how to predict the color of these remnants, and some workarounds.
I modify an earlier particle effect to create colorful, fiery trails behind the particles as they fly around and bounce off circular walls.
A pretty particle effect in HTML5 Canvas with fading trails. We discuss some issues with browser differences, and a resulting solution for consistency across browsers.