Trichromatic vs opponent-process theory: what each explains, and where they meet

The trichromatic theory says that colour vision starts with three types of receptor, each most sensitive to a different part of the spectrum. The opponent-process theory says that the visual system codes colour in opposing pairs: red against green, yellow against blue, and white against black. The two theories do not compete. The three cone types in the retina confirm the trichromatic theory. Later stages compare the cone signals, and the opponent-process theory describes the result. This is why no colour looks reddish green, and why a negative afterimage has the opposite colour.

Most study guides reduce the two theories to two slogans: "red, green and blue cones" and "red–green, blue–yellow, black–white cells". Both slogans are simplifications. Below are the two theories in their authors' own words, the measurements that settled the old dispute, and the details that the slogans leave out.

• Thomas Young proposed three receptor types in 1802. His first choice was red, yellow and blue. In a second paper the same year, he changed this to red, green and violet.
• Ewald Hering based his theory on what colours look like. Red and green never appear together in one colour, and yellow and blue never do either.
• In 1957, Leo Hurvich and Dorothea Jameson combined the two theories: three receptor signals feed three opponent systems. The APA Dictionary of Psychology calls this the dual process theory.
• The real cones are not "red, green and blue" cones. The long-wavelength cone is most sensitive at about 563 nm, which is yellowish-green light.
• The opponent cells that researchers measure do not match Hering's four pure colours exactly. That part of the story is still open.

The difference in one table

Trichromatic theory: three types of receptor

Thomas Young, a British physician and physicist, set out the idea in his Bakerian Lecture, read to the Royal Society in November 1801 and printed in 1802. He argued that each point of the retina cannot contain a separate particle for every possible wavelength. In his words, "it becomes necessary to suppose the number limited, for instance, to the three principal colours, red, yellow, and blue". He added that "each sensitive filament of the nerve may consist of three portions, one for each principal colour" (Young, 1802a, pp. 20–21).

Young changed his choice of colours in a second paper, read in July 1802. After Dr Wollaston's correction of the description of the prismatic spectrum, Young had to modify "the supposition that I advanced in the last Bakerian lecture". He wrote that he was "substituting red, green, and violet, for red, yellow, and blue" (Young, 1802b, p. 395). So the familiar idea of red, green and blue receptors is not Young's first version.

Hermann von Helmholtz championed Young's idea in the 1860 edition of his Handbook of Physiological Optics. This is according to a 2017 tutorial review by Steven Shevell and Paul Martin. The evidence for the theory came from colour matching. Hurvich and Jameson summarised it as "the basic fact of color mixture, namely, that all visible hues can be matched by the mixture, in proper proportions, of only three physical light stimuli" (Hurvich & Jameson, 1957, p. 384).

The theory did not name a cell type at first. The APA Dictionary of Psychology notes that the components "are now thought to be retinal cones, although the original theory was not tied to a particular (or indeed to any) cell type" (APA Dictionary, Young–Helmholtz theory).

What the three cones actually respond to

In 1980, Bowmaker and Dartnall measured the light absorption of single rods and cones from the retina of one man (Bowmaker & Dartnall, 1980). They found three classes of cone.

The numbers show why "red, green and blue cones" is only a label. The L and M peaks are only about 30 nm apart, and the L cone responds most to yellowish-green light, not red. The APA Dictionary says that the peaks "roughly" correspond to blue, green and red (APA Dictionary, trichromatic theory). Researchers prefer the names S, M and L cones. The AP Psychology course description uses the colour names, with the wavelength in brackets: blue (short), green (medium) and red (long) cones (College Board, 2024, p. 44).

Opponent-process theory: colours in opposing pairs

Ewald Hering, a German physiologist, worked in Prague when he developed his theory. Shevell and Martin date it to 1874. Hering collected his papers to the Vienna Academy in the book Zur Lehre vom Lichtsinne (On the theory of the light sense) in 1878.

Hering started from colour appearance, not from colour mixing. He named four chromatic colours that look pure: red, yellow, green and blue. Shevell and Martin explain that no one of these four "unique hues" can be described as a mixture of the others. Orange, by contrast, looks like a mixture of red and yellow. Purple looks like a mixture of red and blue.

But some mixtures never appear. In our translation, Hering wrote that red and green, and also yellow and blue, are "never at the same time clearly noticeable in one colour" (Hering, 1878, p. 109). He called each such pair Gegenfarben, "opponent colours", because of their "hostile" behaviour towards each other. White and black formed the third pair.

Two details in Hering's text are rarely quoted. First, he admitted that he could not say why the pairs exclude each other: "An answer to this is impossible for now." Second, he did not rule out the forbidden mixtures. He thought it "quite conceivable" that someone might one day find conditions that produce a sensation mixed from red and green (p. 110). The section on reddish green below shows that researchers later tried.

In the 1950s, Hurvich and Jameson found a way to measure Hering's pairs. They used a method called hue cancellation. An observer looks at an orange light and adds a green light until the mixture looks "neither reddish nor greenish", as Shevell and Martin describe it. The amount of green light needed is a measure of the red in the orange. With this method, Jameson and Hurvich mapped the red–green and blue–yellow responses of human observers.

How the two theories work together

For decades, the two theories looked like rivals. Shevell and Martin write that Hering and Helmholtz "had fierce disagreements about it, both scientific and philosophical".

Hurvich and Jameson proposed a solution in 1957. Their model has two stages. The first stage has "three independent photosensitive materials", in line with the trichromatic theory. These materials feed three paired opponent systems: yellow–blue, red–green and white–black (Hurvich & Jameson, 1957). The APA Dictionary says: "This combined theory is known as the dual process theory of color vision." It adds that all these theories came "long before" physiologists could record from retinal cells, "yet they were successful in predicting the responses of cells under most conditions" (APA Dictionary, opponent process theory of color vision).

The physiology confirmed both stages. The cone measurements confirmed three receptor types. In the 1960s, Russell De Valois and colleagues recorded single cells in the lateral geniculate nucleus of monkeys (De Valois et al., 1966). This nucleus is a relay station between the eye and the visual cortex. Some cells increased their firing for some wavelengths and decreased it below their resting rate for others. This is an opponent response.

In short, the trichromatic theory describes how the eye takes in colour. The opponent-process theory describes how the next stages code it. Shevell and Martin conclude that "each of their theories captures a fundamental aspect of the human visual system". The AP course description makes the same point in one sentence: "Color vision is explained by both the trichromatic theory and the opponent-process theory."

Where the textbook version is too simple

The textbook version says that the retina has red–green and blue–yellow cells. The real cells follow a slightly different pattern.

In 1984, Derrington, Krauskopf and Lennie tested many colour directions on cells in the monkey lateral geniculate nucleus. The cells fell into two main groups. One group compared the L and M cones (the L–M axis). The other group responded to the S cones against the other two. These are cone-opponent signals. They are not the same as Hering's opponent colours. Shevell and Martin show the result in one figure: the unique hues that observers chose "do not align with the cardinal color axes" of these cells. The observers also disagreed with each other. Their choices of unique yellow were close together, but their choices of unique blue, red and green were spread widely.

So the cells near the start of the pathway compare cones, and Hering's pairs describe how colours look. The link between the two is not settled. Shevell and Martin write that "perceptual and physiological opponency need not converge at the level of individual highly tuned nerve cells". For an exam, the AP wording is the expected answer. For a real understanding, "red–green cell" is a short name for a cell that compares L and M cones.

What afterimages show

Afterimages are the standard evidence for the opponent-process theory. Look at a bright coloured patch for about 30 seconds, then look at a white surface. You see the same shape in the opposite colour. The APA Dictionary says that a negative afterimage is "usually complementary to the original stimulus in color and brightness; for example, if the stimulus was bright yellow, the negative afterimage will be dark blue" (APA Dictionary, afterimage).

The opposite colour fits the opponent pairs. After a long look at yellow, the yellow–blue system adapts to yellow. When the yellow is removed, the balance tips towards blue. Our article on sensory adaptation explains how a sensory system responds less to a stimulus that continues.

Many popular explanations say that the cones get tired. That explanation is incomplete. In 2012, Qasim Zaidi and colleagues recorded from retinal ganglion cells in monkeys, the cells whose fibres form the optic nerve (Zaidi et al., 2012). They note that proposed explanations "range from bleaching of cone photopigments to cortical adaptation". In their recordings, all three classes of ganglion cell adapted to long stimuli, "with much slower time constants than those expected of photoreceptors". When the stimulus stopped, the cells gave a rebound response. The authors concluded that "afterimage signals are generated in the retina but may be modified" by the cortex. This finding agrees with the AP description, which links afterimages to ganglion cells, not to the cones alone.

What colour blindness shows

Colour vision deficiency is common. According to the National Eye Institute, "about 1 in 12 men" have it, and men have a much higher risk than women. The most common type makes red and green hard to tell apart. Another type makes blue and yellow look the same. Complete colour blindness is rare. Most people are born with the condition. Colour vision can also change later in life. The NEI lists eye disease, damage to the optic nerve or brain, and some medicines as possible causes, so an eye doctor should check such a change.

The two theories explain different parts of colour blindness:

• The cause usually sits at the cone level. In dichromatism, the eye has only two of the three cone photopigments. In deuteranopia, the green-sensitive (M) photopigment is missing. In protanopia, the red-sensitive (L) photopigment is missing, and red light looks very dim (APA Dictionary, dichromatism). This is the trichromatic theory's level.
• The pattern of confusion follows the opponent pairs. People with these types confuse red with green, the two members of one pair. Hering himself explained colour blindness as the absence of one or more of his chromatic processes (APA Dictionary, Hering theory).

The chemist John Dalton gives a famous example. He described his own colour blindness in 1794. He confused scarlet with green and pink with blue. Dalton thought that the fluid in his eyes was tinted blue, and he asked for his eyes to be examined after his death. The fluid was clear. In 1995, David Hunt, John Mollon and colleagues extracted DNA from his preserved eye tissue. Dalton was a deuteranope, so his M cone photopigment was missing. The result also showed that Thomas Young had been wrong: Young believed that Dalton was a protanope (Hunt et al., 1995).

Can anyone see a reddish green?

The opponent-process theory says that reddish green and yellowish blue are impossible colours. Researchers call them forbidden colours. Hering thought that a reddish-green sensation was conceivable, and some experiments say that people can see one.

In 1983, Hewitt Crane and Thomas Piantanida showed observers a red stripe next to a green stripe. Their equipment held the image of the border between the stripes still on the retina, but not the outer edges. Under these conditions, the region could be "perceived simultaneously as both red and green" (Crane & Piantanida, 1983). In 2001, Vincent Billock and colleagues noted that in the first study "only some observers saw forbidden colors". With colours of equal brightness, their own observers reported reddish greens and bluish yellows (Billock et al., 2001).

Other researchers disagree. In 2006, Hsieh and Tse found that the colours mixed even when each colour went to a different eye. They concluded that this illusory colour mixing "does not result in 'forbidden colors'" (Hsieh & Tse, 2006). The question is still disputed. Under normal viewing, the opponent rule holds.

Not the same as the opponent-process theory of emotion

The same name refers to a second, unrelated theory. In 1974, the psychologists Richard Solomon and John Corbit published "An opponent-process theory of motivation" in Psychological Review (Solomon & Corbit, 1974). The theory is about feelings, not colours.

According to the APA Dictionary, the theory says that a stimulus causes a primary feeling, pleasant or unpleasant, and also an opposite feeling. The opposite feeling starts late and fades slowly, so it is the stronger feeling for a time after the stimulus ends. It also gets stronger with repetition. Solomon used the theory to explain motives such as drug use, love and social attachment (APA Dictionary, opponent-process theory of acquired motivation). On an exam, the context tells you which theory the question means: wavelengths and afterimages point to Hering, and drug use, love or social attachment point to Solomon.

Keeping the terms apart

Colour vision has many terms that are easy to mix up: trichromatic theory, Young–Helmholtz theory, opponent-process theory, unique hues, dual process theory, negative afterimage, dichromatism, deuteranopia and protanopia. In Psychology World you can save terms like these to one list. Then you can drill the list as flashcards, as multiple-choice questions once it has four or more terms, or by typing the answer. What each study mode does is set out on the home page. Our guide to memorising psychology terminology explains which study methods have the strongest evidence.