Color Mixing

Color Mixing

Light from red, green and blue LEDs is projected into overlapping circles. The intensity of each LED light can be adjusted to make white or other colors in the overlapping region.

Spectral colors are characterized by the wavelength of the electromagnetic radiation. The shortest wavelength that can be seen by the eye is 380 nm violet. The longest wavelength which can be seen is 770 nm red. The wavelengths of the light produced by the red, green, and blue LEDs is shown in the top graph, along with that of an amber LED.

One interesting thing to note is that the spectrum of the amber LED does not overlap very much with the spectrum of the red and green LED. The light from the red and green LEDs does not contain any amber wavelengths around 590 nm, yet we see the color amber when we mix red and green light. Why is this?

The answer can be found in the sensitivity of the three types of cones in most peoples eyes (see the second graph). These are sensitive to short, medium, and long wavelengths which roughly correspond to blue, green and red light. Amber light at 590 nm, excites both the green and red cones. We can fool the brain into seeing amber, by supplying the right amount of red and green stimulation. That's why color mixing works. However, everyone's eyes are different. Some people only have two types of cones and have some color blindness. Some reportedly have four types. The ratio of red to green cones varies widely from person to person. The ratio of red and green light to match the spectral 590 nm amber also varies from person to person. We don't all see the same colors.


RGB display devices like TV's and monitors project red, green, and blue light to produce the sensation of all the colors. The CIE diagram shows all the colors that can be perceived by the eye/brain. The spectral colors lie on the white curve identified by wavelength and all the various mixtures appear inside. The color gamut of a display device is that part of the full range of colors that the device can reproduce. The color gamut of the red, green and blue LEDs is the area inside the triangle with vertices at the wavelengths of the LEDs. Colors outside the triangle cannot be matched by mixing the light of those LEDs. How close can we get to spectral amber?

Printer inks, paints, and filters use subtractive rather than additive colors. Instead of red, green, blue, printer primaries are usually cyan, magenta, yellow, and black. Printers also have a color gamut and can't reproduce colors outside their gamut. This can lead to problems when the display gamut doesn't match the printer gamut. Colors that can be seen on the screen can't be printed.