If you do not want the colours to mix, be sure to wait for the first layer to dry before applying another colour over it or up against it.
Hint: Regularly change the water you rinse your brushes in and preferably work with two water bowls: one for light and one for dark shades.
Grimas Water Make-Up (Pure) can be used around the eyes, but children often do not like it when brushes are so close to the eyes. So decide yourself how far you want to extend the colour.