Many of these links are for use with children but may provide a starting place for economical do it yourself paints to practice with as you learn. It might also entertain children in the process!
Homemade Watercolor Paints from Flowers.
DIY recipes for painting and art for children. Also, here. This one shows a wider range of possible mediums as well.
Frida Kahlo's DIY Paint Recipe
Homemade salt and flour paint suitable for children