The colors don't mix, but rather stay separate. You can remove the paint and start over by laying newspaper on the surface to pick up all the paint from the surface. With all the desired colors of paint on the surface, use a toothpick, a comb or straws, etc. to carefully create swirls and design patterns. Dip a whisk or a bundle of broom straws to flick drops onto the surface. The droplets give a different effect.
Most books suggest using distilled water.
That could mean a lot of bottled water to buy. Try it first with tap or filtered water. Hard water will interfere with the process. This will wash out anything that is likely to prevent the paint from sticking. Cool to room temperature.