We all recognize the work of 20th Century Modernist, Piet Mondrian. This unit borrows from his work directly. It is a 2D composition using only the primary colours plus black, squares and rectangles cut from construction paper and glued onto a white background (cartridge paper). Plus, there are a few extension ideas for higher learners included.
Have a look at my other lesson that connects Math (Geometry) and the Golden Ratio
Principles of Design: Using the Golden Ratio as Art,