Working initially with materials that included metallic paints, and later with interference pigments, his paintings shimmer and alter in the light, depending on their angle of view. Simpson has exhibited his work widely since the 1950s, throughout Europe, Asia, and the United States. He is represented in public and private collections including the Panza Collection, Varese, Italy; the Museo Cantonale d’Arte, Lugano, Switzerland; the Museum of Modern Art, NY; San Francisco Museum of Modern Art, CA; Philadelphia Museum of Art, PA; Seattle Museum of Art, WA; National Collection of Fine Art, Washington, DC; John D. and Catherine T. MacArthur Foundation, Chicago; and the Contemporary and Modern Art Museum of Trento and Rovereto, Italy. Simpson received his MFA from San Francisco State College (now SFSU) and BFA from the California School of Fine Arts (now San Francisco Art Institute).