"Theory on Colors," or "Zur Farbenlehre" in its original German, contains three sections: the didactic section in which Goethe describes his observations regarding color, the polemic section in which he argues against Newton's theory of optics and the historic section in which he discusses other pertinent theories. It was very influential to the Pre-Raphaelite art movement.
Goethe used turbidity to conduct experiments in color. He noticed how the different colors appeared closer to light and dark depending on how dark or light the medium was. Light going through a dark medium had hues of yellow; darkness seen through a light medium had hues of blue. He concluded that color was the result of light interacting with shadow, or darkness.
Goethe said that perception is paramount when viewing color. When you project a shaft of light through a prism, the color you see depends entirely on if the light is on the top or the bottom. If the light is on top, you see the blues and violets bordering the dark edge, and if the light is on the bottom, you see the reds and yellows bordering the light edge. You can produce the entire spectrum of light by shining a beam of light through a prism in a dark room or projecting a shadow through a prism in a dark room. In the prior, the overlap appears green, and in the latter, it appears magenta.
It has been said that Goethe and Newton, who was not Goethe's contemporary but rather his predecessor, were at odds when it came to optics; this is not entirely true. Goethe simply saw things differently from Newton. Newton was a scientist, and his theory on optics presented it as such. Newton saw light as containing all colors and darkness as being the absence of color. He claimed that color depends on the object, the light and the mechanisms behind human vision. Goethe saw color as symbolic, almost religious. It was affected not just by light and objects, but also by human perception.
Most physicists dislike Goethe's theory of color. In fact, they tend to dismiss it altogether. Advances in science and optics have rendered many of Goethe's experiments invalid and out of date, but he is not criticized for his methods. The scientific method is, after all, based on conducting experiments to test hypotheses set forth by observations. This is exactly what Goethe did.