Piet Mondrian used only the primary colors of red, yellow, and blue in his abstract paintings because he felt they created the strongest contrasts. His paintings featured squares and rectangles of these colors arranged in vertical and horizontal lines. Contrast refers to the arrangement of opposite elements like light and dark colors, large and small shapes, or smooth and rough textures to create visual interest and drama in a work of art. The document instructs students to recreate Mondrian's style using rulers, markers, and the primary colors on paper by drawing lines to divide it into squares and coloring them in.