If you want smaller file sizes, just ensure your artwork is 256 color or less and C2 will automatically make it a palette PNG on export (blog post about it). HTML5 doesn't actually natively support palettes, I think the demo you linked to is a very cleverly prepared demo where you split the images in to red, green and blue channels and blend them with different opacities or something like that. So you can't actually switch a palette like the old 8 bit games used to, but you might already be able to fake it by preparing your images and using effects in C2.