To train my ability to see clearly and also pigments mixture reflexes, I bought few blocks, painted them with poster colors. I put them in different lighting situation and try to reproduce them. I am ashamed to say that my actual difficulty was to draw those cube solidly. They always end up being skewed, different size, or floating around. I really like the exercices, the simplicity of it make mistakes much more visible. Another good point is you dont spent too much time drawing, but more time is spent focusing on colors and mixing them. I went back to the store to get donut, cones and cylinder, I will paint them with secondary colors.