Yellow is a primary color that can be produced by mixing other colors of light together. The two colors that can combine to create yellow are red and green.
How Light Colors Mix
The primary colors of light are red, green, and blue. When all three combine together, they make white light. When any two primary colors mix, they produce a secondary color.
Here are the secondary colors that are made when two primary colors combine:
| Primary Colors | Secondary Color |
|---|---|
| Red + Blue | Magenta |
| Green + Blue | Cyan |
| Red + Green | Yellow |
So when red light and green light mix together, they produce the secondary color yellow. This is why yellow can be considered a “derivative” color rather than a primary color in the world of light.
The Additive Color System
The mixing of colored lights works through the additive color system. This means that as more colors are added together, they move towards white light.
Starting with no light at all, which is black, as each primary color is added, they combine to make new colors:
- Red + Green = Yellow
- Red + Blue = Magenta
- Green + Blue = Cyan
- Red + Green + Blue = White
In this additive system, yellow is a brighter color than either red or green on their own. When the two mix, the red light and green light combine to excite the cones in our eyes that detect these wavelengths. The combination is interpreted by the brain as the color yellow.
Pigments vs. Light
It’s important to note the distinction between colored light and colored pigments. With pigments, such as paint, ink, or dyes, the primary colors are red, blue, and yellow. Combining all three makes black.
This is known as the subtractive color system, as added pigments subtract wavelengths of light to make darker colors. But when working with light itself, the primary colors are red, green, and blue, and combining these makes lighter colors including yellow.
Color Perception
Our eyes contain photoreceptor cells called cones that detect different wavelengths of light. There are cones specialized for red, green, and blue wavelengths. All other colors we perceive are combinations of signals from these three cone types.
When both the red and green cones are stimulated, the brain perceives the color yellow. Even though yellow light has a distinct wavelength range of 570-590 nm, we can’t detect this single wavelength on its own. Yellow is constructed in our brain based on the combined input from the red and green cones.
Yellow in Nature vs. Tech
In nature, yellow light comes from the sun. The sun emits a continuous spectrum of all visible wavelengths of light. The yellow portion of this spectrum has a wavelength range falling between red and green.
But man-made yellow light is produced by combining solid-state red and green light sources like LEDs (light emitting diodes). These discrete red and green wavelengths are mixed to stimulate the perception of yellow in our visual system.
True or Pure Yellow
Is there such a thing as true or pure yellow light? In theory, light of 580 nm would stimulate the eye in the most pure or optimal way to produce the perception of yellow. But in reality, we can’t isolate a single wavelength. Natural and artificial light sources emit a range of wavelengths.
So while a narrow band of yellow light centered on 580 nm would be considered the purest or truest yellow, we can effectively make this color by blending red and green, which together cue the eye and brain to see yellow.
Shades of Yellow
Not all yellow light looks the same. By varying the intensity and ratio of red to green, different shades of yellow can be produced.
Here are some examples of shades of yellow from light:
- Reddish yellow – more intense red
- Greenish yellow – more intense green
- Pale yellow – less saturated red + green
- Golden yellow – more saturated red + green
When working with lighting instruments like theater gels or photography filters, these different shades of yellow can be created by using graduated filters or adjusting color intensity.
Other Ways to Make Yellow
While combining red and green light is the primary way of making yellow, here are a few other ways yellow can be produced with light:
- Mixing a blue light with a broadband yellow filter will transmit only yellow wavelengths.
- Using a prism to separate white light into a rainbow spectrum will reveal the yellow band of wavelengths.
- Shining sunlight through a yellow solution will preferentially transmit yellow light.
- Using a laser or other monochromatic light source tuned to 580 nm yellow.
So in summary, while pure yellow light can theoretically exist at around 580 nm, most yellow light we encounter is a mixture of red and green wavelengths stimulating our eye’s color receptors.
Conclusion
When red and green light mix together, they produce the color yellow through the additive color system. This is because the combination of red and green wavelengths stimulates the cones in our eyes responsible for the perception of yellow. Though true yellow light exists at 580 nm, most yellow light involves a blend of longer red wavelengths and shorter green wavelengths combining to create the various shades of yellow we see.