Parrish's art features dazzlingly luminous colors; the color Parrish blue was named in acknowledgement. He achieved the results by means of a technique called glazing where bright layers of oil color separated by varnish are applied alternately over a base rendering (Parrish usually used a blue and white monochromatic underpainting).
He would build up the depth in his paintings by photographing, enlarging, projecting and tracing half- or full-size objects or figures. Parrish then cut out and placed the images on his canvas, covering them with thick, but clear, layers of glaze. The result is realism of elegiac vivacity. His work achieves a unique three-dimensional appearance, which does not translate well to coffee table books.
The outer proportions and internal divisions of Parrish's compositions were carefully calculated in accordance with geometric principles such as root rectangles and the golden ratio. In this Parrish was influenced by Jay Hambidge's theory of Dynamic Symmetry.
Parrish devised many innovative techniques. A technique which Parrish used frequently involved creating a large piece of cloth with a geometric pattern in stark black-and-white (such as alternate black and white squares, or a regular pattern of black circles on a white background). A human model (often Parrish himself) would then pose for a photograph with this cloth draped naturally on his or her body in a manner which intentionally distorted the pattern. Parrish would develop a transparency of the photo, then project this onto the canvas of his current work in progress. Using black graphite on the white canvas, Parrish would painstakingly trace and fill in all the black portions of the projected photo. The result was astonishing: in the finished painting, a human figure would be seen wearing a distinctive geometrically patterned cloth which draped realistically and accurately.