In the late 1860s, through the practice of painting light and water "en plein air" (in the open air), he and his friend Claude Monet discovered that the color of shadows is not brown or black, but the reflected color of the objects surrounding them, an effect today known as diffuse reflection. Several pairs of paintings exist in which Renoir and Monet, working side-by-side, depicted the same scenes ("La Grenouillère", 1869). 