Trycolors.com is a very useful website for mixing colors. It is great for making new colors for soaps, eyeshadows, nail polishes, lipsticks or other color cosmetics.
There are couple ways you can use this website. First one is that you can choose any shade from the palette (thousands of colors to choose from) and it will show you what colors and with what proportions you need to mix to get that shade.
And lastly, you can just go there and start mixing colors on your own. For example, if you own red and blue pigments you can experiment with mixing these two colors to see what shades you can get. This simple website saved me lots of money that I would otherwise have spent experimenting with real pigments.