Drag the pieces to the canvases to create Mondrian like paintings.
To change color, click on the icon on the top right, that has a circle and a triangle.
You can rotate the pieces by dragging on the little circle on one of the corners.
Copy and paste (Crtl+C, Ctrl+V) to make more copies of a piece.
If you need other pieces (other rectangles that are not listed), use the tool with the triangle and the arrow. Select the tool and then click on the four corners of your rectangle (in order), click on the first corner again to close the shape.