In the late 1860s , through the practice of painting light and water `` en plein air '' ( outdoors ) , 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 known today as diffuse reflection .
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 .