This online color mixing tool lets you experiment with mixing primary colors and secondary colors without worrying that you're wasting paint. While the onscreen colors don't mix exactly as paint does, it'll give you the idea of what'll happen.
Simply click on a color you want to add to the mix. One click adds a fixed quantity of "paint" to the mixing area, which changes to reflect the new mixture. The numbers on the colors show the proportion of that color in the mix. To start again, click on the reset button or in the mixing area.