The color and patterns seen on most butterflies are created by the overlapping scales and the color of the pigment on each scale. In the case of the Emerald Swallowtail, the scales on the inside of the wings are like small prisms which make the color iridescent. By looking from different angles, a change in color can be observed." reimangardens

Other Examples: http://www.pbase.com/britestar/image/163893875 and http://www.pbase.com/britestar/image/163895283