Questions: Color Vision: Three Cone Types and Trichromatic Perception

Question 1

A TV screen produces 'yellow' by mixing red and green pixels — it emits no actual yellow wavelength (~580 nm). How does trichromatic theory explain why this still looks yellow?

• A. The brain averages red and green signals and outputs the intermediate color yellow
• B. The red and green pixel mixture produces the same M:L:S cone activation ratio as a true 580 nm yellow wavelength, so the brain computes the same color percept
• C. The S-cone responds to the combination of red and green and generates the yellow sensation
• D. Human eyes cannot distinguish mixed wavelengths from pure wavelengths, so all mixtures look monochromatic

Answer: (B)

This is metamerism — the same perceived color from physically different spectra. Because the visual system only has three cone types and color is computed from their ratio, two different spectral distributions that produce identical M:L:S ratios are visually indistinguishable. A 580 nm yellow wavelength and a red+green mixture can produce the same cone ratios, yielding identical yellow percepts. Color is a computation over cone ratios, not a measurement of wavelength.

Question 2

A person loses their M-cone photopigment due to a genetic deletion. What is the most accurate prediction about their color vision?

• A. They see the world entirely in black and white, because color perception requires all three cone types
• B. They retain color vision but lose the ability to distinguish colors that differ primarily in the red-green dimension, because the red-green opponent channel collapses
• C. Their blue-yellow sensitivity increases to compensate for the lost cone type
• D. They can only see primary colors (red and blue), since only L-cones and S-cones remain active

Answer: (B)

Losing M-cones collapses the red-green opponent channel (which compares L and M signals) but leaves the blue-yellow opponent channel (S vs. L+M) and the luminance channel intact. The person still experiences color — they can distinguish blue from yellow — but cannot distinguish colors that differ only in the L-M ratio (e.g., red from green, orange from khaki). This partial color blindness confirms that color vision is organized around opponent channels, not individual cones.

Question 3

Two physically different light spectra can produce identical color sensations if they generate the same ratio of activation across L-, M-, and S-cones.

• T. True
• F. False

Answer: True

This phenomenon, metamerism, is a direct consequence of trichromacy. The visual system has only three channels (cone types), so it cannot distinguish spectra that happen to produce the same three responses. Metamers — physically distinct lights that look identical — are only possible because color perception is a three-dimensional reduction of the continuous spectrum. This principle underlies all color reproduction technology: monitors, printers, and photography all exploit metamerism to reproduce colors using limited primaries.

Question 4

Each cone type signals a specific color — L-cones signal red, M-cones signal green, S-cones signal blue — and color perception is just reading out which cone fired most strongly.

• T. True
• F. False

Answer: False

This is the most common misconception about color vision. Each cone responds to a broad range of wavelengths, and their sensitivity curves overlap substantially. A single wavelength activates all three cone types to different degrees. Color is computed by comparing the ratio of responses across all three types — not by reading a single cone's output. The brain interprets relative activation patterns, not absolute signals. Opponent processing in retinal ganglion cells explicitly encodes differences between cone responses, not individual cone signals.

Question 5

Why does trichromacy explain how a screen with only red, green, and blue pixels can reproduce the full range of perceived colors?

Model answer: Because color perception depends on the ratio of activation across three cone types, not on which specific wavelengths are present. By mixing red, green, and blue light at the right intensities, a screen can match the exact L:M:S cone activation ratios produced by any natural color. When the ratios match, the brain computes the same color percept — the difference in physical spectrum is invisible. This works precisely because color is a three-dimensional computation, and three primaries are sufficient to span that space.

The three-dimensionality of human color vision is not a limitation — it is the mathematical fact that makes color reproduction technology possible. Any color can be specified by three numbers (its L, M, S cone activations), and any combination of three independent primaries can reach any point in that three-dimensional space. More cone types would require more primaries; fewer would reduce the gamut. The trichromatic structure of human vision is exactly matched by the RGB architecture of displays.