Yellow is a primary color that appears between red and green on the visible spectrum. Many people believe that yellow is a secondary color created by mixing red and green light together. However, the relationship between yellow, red, and green is more complex than simple color mixing.
The perception of yellow involves specialized retinal cells in the eye called cone cells. There are three types of cone cells that detect different wavelengths of light – those that are most sensitive to red, green, and blue wavelengths. The combination of signals from these three cone types allows us to perceive the range of colors we see.
So is yellow just a combination of red and green cone cell signals? Or is it detected by a unique set of cells in the retina? Understanding the neuroscience and physics behind yellow perception can help answer this question.
The Visible Spectrum
The visible spectrum is the range of wavelengths of electromagnetic radiation that humans can see. It spans roughly 400-700 nanometers (nm) in wavelength. The longest wavelengths appear red, transitioning through orange, yellow, green, blue, and violet as the wavelengths get shorter.
| Color | Wavelength range (nm) |
|---|---|
| Red | 620-750 |
| Orange | 590-620 |
| Yellow | 570-590 |
| Green | 495-570 |
| Blue | 450-495 |
| Violet | 380-450 |
Yellow light has wavelengths between 570-590 nm, placing it between the red and green portions of the spectrum. This proximity to red and green light leads many to believe yellow is a mix of those two colors. However, the perception of yellow has more to do with the biology of human color vision.
Trichromatic Color Vision
Human color vision relies on specialized photoreceptor cells in the retina called cone cells. There are three types of cone cells sensitive to different wavelengths of light. According to the trichromatic theory of color vision, all color perceptions depend on signals from these three cone types:
- L cones – Most sensitive to red, long wavelength light (peak around 560 nm)
- M cones – Most sensitive to green, medium wavelength light (peak around 530 nm)
- S cones – Most sensitive to blue, short wavelength light (peak around 420 nm)
The L, M and S cones do not precisely correspond to seeing pure reds, greens and blues. Rather, their signals are compared by downstream visual processing to produce red-green and blue-yellow opponent mechanisms in the visual system. It is the ratio of activity between different cone types, not the absolute activity levels, that allows us to perceive color.
Opponent Color Theory
Opponent color theory states that certain colors cannot be perceived together. There are two opponent mechanisms:
- Red-Green – Red and green cone signals are opposed and processed by different neural pathways.
- Blue-Yellow – Blue and yellow are opposed and processed separately.
This theory explains why we do not see reddish-greens or bluish-yellows. The boundaries between red, green, blue and yellow are sharpened by the opposition built into visual processing.
Where does yellow fit into this scheme? Yellow is thought to rely on a relatively high L cone (red) response combined with a low-moderate S cone (blue) signal. The absence of M cone (green) activity differentiates yellow from red. Yellow perception relies on an L-M opponent mechanism, distinguishing it from both red and green.
The Special Case of Yellow
From the opponent process theory, yellow occupies a special position in color vision. It is not solely a mix of red and green signals, but rather a distinct channel with relationships to both red and green mechanisms.
Several lines of evidence support yellow having a unique perceptual process:
- S cone (blue) signals modify yellow perception, shifting it towards green. This shows yellow relies on more than just L and M cones.
- Retinally stabilized yellow targets – keeping an image fixed on the retina – appear to fade or turn brown. This does not happen with red or green targets, indicating yellow has distinct processing.
- Electrical stimulation of the visual brain can produce a sensation of yellow without activating L-M opponent pathways. This points to dedicated yellow circuitry.
- Color appearance models assign yellow its own color dimension between red-green and blue-yellow axes.
Additionally, yellow has the highest luminance of all colors for a given level of cone stimulation. This may be an adaptive advantage to discern yellows efficiently. Together, these findings suggest yellow has specialized neural mechanisms beyond a simple mixing of red and green.
Color Mixing with Pigments vs. Light
Part of the confusion around yellow’s relationship to red and green stems from differences between color mixing with pigments versus light. The distinction is important:
- Pigment mixing (paints, dyes) involves substances selectively absorbing certain wavelengths of light and reflecting the remaining wavelengths. Mixing red and green pigments together produces a yellow color by reflecting both red and green light.
- Additive light mixing involves shining different colored lights together. But mixing red and green light actually produces yellow light with a unique wavelength, not a simultaneous presence of red and green wavelengths.
So additive mixing of red and green light can produce the sensation of yellow. But this is still distinct from reflecting red and green wavelengths simultaneously off a yellow surface. Yellow light stimulates the eye in a different way than seeing red and green light together.
Cone Stimulation by Yellow Light
What happens when yellow light enters the eye? Being midway between red and green wavelengths, it provides moderate stimulation of both L and M cones:
| Cone Type | Stimulation by Yellow Light |
|---|---|
| L cones (red) | High |
| M cones (green) | Moderate |
| S cones (blue) | Low |
This pattern of cone stimulation does not match seeing pure red and green light simultaneously. The latter would produce high L cone activity and high M cone activity concurrently.
Yet objects reflecting yellow light into our eyes evoke the perception of yellow, not red + green. This demonstrates yellow color vision relies on specialized retinal and brain mechanisms, not just mixing L and M cone signals.
Color Mixing Summary
To summarize the complex relationship between yellow, red and green:
- Additive mixing of red and green light can create yellow light wavelengths.
- But this yellow light has a distinct cone stimulation profile, different than concurrent red + green.
- The perception of yellow relies on retinal and neural processing distinct from red and green.
- Mixing red and green pigments creates a surface reflecting both red and green light, different than pure yellow light.
- Yellow occupies a unique position between the red-green opponent mechanisms in color vision.
So while yellow is close to red and green along the visible spectrum, it is not solely a mixture of those two colors in the human visual system.
Yellow in Color Vision Deficiencies
Further evidence that yellow perception is unique comes from studies of people with color vision deficiencies:
- Protanopes (lacking L cones) have difficulty discriminating yellows, but can still perceive them. This indicates yellow perception does not solely require L cones.
- Deuteranopes (lacking M cones) can discriminate yellows well, even in the absence of green perception.
- Tritanopes (lacking S cones) have trouble discriminating blues and yellows, suggesting S cones help modulate yellow perception.
These findings show yellow color vision cannot be explained by a simple mixing of signals from L and M cones. The involvement of S cones and additional neural processing is required to account for yellow sensation.
Color Appearance Models
Sophisticated models of color vision account for the special nature of yellow within the red-green opponent dimension of color space.
For example, the Natural Color System (NCS) represents color as a ratio of two chromatic components and blackness/whiteness. Yellow has both high redness and low greenness, located between red and green hues. Moving vertically on the NCS yellow axis corresponds to changes in lightness and saturation.
The widely used CIELAB (L*a*b*) color space also assigns yellow a unique position. It is modeled along the b* axis between red-green and blue-yellow coordinates.
These models demonstrate that a dedicated yellow component is needed, in addition to the standard red-green and blue-yellow axes, to account for the full gamut of human color vision.
Yellow and the Retina
In addition to the cone cells, the retina contains other cells involved in processing color vision signals:
- Horizontal cells help compare and contrast L and M cone signals.
- Ganglion cells integrate inputs from cones and other retinal neurons.
- Some ganglion cells respond selectively to combinations of L and M cone input that correspond to yellow hues.
This points to substantial retinal circuitry dedicated to encoding yellow, prior to the visual signals reaching the brain.
Interestingly, a small percentage of women have an extra type of cone cell with peak sensitivity around the yellow portion of the spectrum. The role of these rare tetrachromatic visual systems in yellow perception remains under study.
Neural Processing of Yellow
At higher stages of visual processing, yellow color perception depends on neural activity patterns within the visual cortex of the brain.
Using brain imaging and direct neuronal recordings, scientists have found:
- Neurons in visual area V4 are specialized for the perception of yellows, distinct from red and green neurons.
- The posterior inferior temporal cortex contains populations of neurons selective for different yellow hues.
- Lesions to visual cortex can impair yellow perception while sparing ability to see red and green.
This demonstrates substantial neural machinery devoted to yellow vision, not simply red-green mixing. The circuitry spans from the retina upstream through multiple stages of visual cortex.
Conclusion
While yellow occupies a middle ground between red and green in the visible spectrum, human color vision employs specialized mechanisms to perceive yellow, from the retina upstream through the visual pathways in the brain. Yellow perception cannot be explained by a simple mixing of “red” and “green” visual signals.
The complex neuroscience of yellow sensation relies on:
- L, M, and S cone interactions
- Retinal processing of cone signals
- Opponent color pathways
- Unique neural populations responding to yellow hues
So in summary, while additive mix of red and green light can create yellow wavelengths, the perception of yellow involves substantial visual processing beyond mixing red and green color channels. Yellow occupies a distinct slice of color vision, between red and green but requiring specialized retinal and cortical mechanisms to achieve its vivid appearance.