Did you realize that our kids learn colors theory in the very young age? They understand that red and blue make purple by the time they are 4 years old. How do we build on this very basic knowledge to show them how many different colors and shades they can create and use in their art? First, you have to start from removing multi-color paints set. We all relay on them too much that is for sure. Then you need to buy (or make) a color wheel with a basic description on how to use it and make the best out of it. After that only one and the most important is left, practise, practise, practise. There is nothing better like trying and mixing different proportions of different colors to experience what results you can achieve.
If you are working on some particular painting it is good to take a note how you have achieved some color or shade and then paint because this is what you are best at! My class started with practice sketch of landscape which they have imagined or looked at from the screen of iPad. When they were happy if their design picture got transferred to the canvas. Then we got on with mixing of colors and creating different values to differentiate between different parts of the landscape. Color theory is such a brilliant part of technical skills set which students have to learn. You could really practice on any type of subject and with any type of medium however it is much easier to introduce it to kids with acrylic or oil paints.