Every painting lesson I teach begins with this sort of color exploration. Because these children are 4th graders I will not only teach secondary colors, but also make them aware of how they can manipulate the mount of paint they add to the mix to create a variation of the secondaries. I explain that experts in any media are not born, but that only in time spent working with the media does one become more adept at manipulating the desired affects.
Below are what I call 'mixing papers'. Since I try to use the same palettes many times over, it's imperative that they NOT mix their colors on the 6-color palettes I give them. They combine their colors either on their own work or on these small mixing papers to see what new colors are created. Through time and practice children learn to, in advance, know how to make the colors they need just from switching around the color recipes.