Color Mixing

Color addition is a fundamental concept in color theory, which encompasses color mixing, additive and subtractive synthesis, as well as color perception.

Discover FizziQ

How to measure it in class

FizziQ’s colorimeter measures the red, green and blue components of the light received by the camera: it makes it possible to verify, with figures to back it up, what a mixture of lights produces and what a mixture of filters produces.

Steps:

Open FizziQ and select the Colorimeter instrument. Aim at a white area of a screen and record the reference (R, G, B) triplet.
Display pure red, pure green and pure blue in turn on a second screen and measure the three components each time.
Superimpose two flashlight beams fitted with red and green filters on a white sheet, then aim at the overlap zone: the colorimeter reads yellow. This is additive synthesis.
This time stack the same two filters one behind the other in front of a single lamp and aim at the transmitted light: it collapses toward black. This is subtractive synthesis, with the same equipment.
Repeat with cyan and yellow filters stacked together: the transmitted light is green, whereas two superimposed cyan and yellow lights would give a pale white.
Compare the two series of measurements and state the rule: adding lights brightens, stacking filters darkens.

Scientific activities on this topic

Additive RGB color mixing - measure with the colorimeter the colors obtained by superimposing red, green and blue lights, and recover the secondary colors cyan, magenta and yellow (middle school).
Colorimetry of autumn leaves

Learn more

Color addition is a fundamental concept in color theory, which encompasses color mixing, additive and subtractive synthesis, as well as color perception. This article explores these concepts and their application in fields such as painting, graphic design and digital colors.

1. Primary and secondary colors

Primary colors are the fundamental colors from which all other colors can be obtained by mixing. In additive synthesis, the primary colors are red, green and blue (RGB). Subtractive synthesis uses cyan, magenta and yellow (CMY) as primary colors. Secondary colors are obtained by mixing two primary colors.

Caution: the subtractive primaries really are cyan, magenta and yellow. The formulation “red, blue and yellow”, still taught in art classes, is an old simplification that is wrong in physics: with that trio you cannot obtain light, saturated hues, and that is why printers use cyan, magenta and yellow cartridges, never red and blue.

2. Additive and subtractive synthesis

Additive synthesis concerns the mixing of colored lights, used in display technologies (computer screens, televisions, video projectors). Light is added to light: the mixture is therefore always brighter than each of its components. When two RGB primary colors are mixed, they produce a secondary color: red and green give yellow, red and blue give magenta, and green and blue give cyan. The three primaries combined at full intensity give white.

Subtractive synthesis applies to pigments and filters, which absorb certain wavelengths of white light and let through or reflect the others. Each added pigment or filter removes light: the mixture is therefore always darker. By mixing CMY primary colors, secondary colors are obtained: cyan and magenta give blue, cyan and yellow give green, and magenta and yellow give red. The three primaries combined give black - in practice a very dark brown, which explains the additional black ink cartridge in printers, hence the acronym CMYK.

These two syntheses are not two conventions to choose from: they describe two different physical situations. That is why blue and yellow give green in painting - the blue pigment absorbs red, the yellow pigment absorbs blue, only green remains - whereas two superimposed blue and yellow beams give white, since blue and yellow are complementary colors in light.

3. Complementary colors and color harmony

Complementary colors are pairs of colors that, when mixed, produce a neutral color (gray or white). On the color wheel, complementary colors are located opposite each other. For example, red is complementary to cyan, green to magenta, and blue to yellow.

Color harmony is the combination of colors that work well together and are visually pleasing. Harmonious color palettes often use complementary colors or neighboring colors on the color wheel.

4. Application in graphic design and digital colors

In graphic design, understanding color mixing and harmony is essential for creating attractive visuals. The additive color method is used in many fields that use light sources, such as computer displays, televisions, projectors, traffic lights, vehicle lights and stage lighting. Colors are typically created using light-emitting diodes (LEDs) or liquid crystals (LCD) that can produce different colors by adjusting the proportions of red, green and blue light. Subtractive synthesis is used for printing. Color perception by the audience is also crucial, as it influences how people react to a design.

5. Orders of magnitude

A screen encodes each component on 8 bits, that is 256 levels, which gives 256³ ≈ 16.7 million displayable colors. A smartphone screen sub-pixel measures a few tens of micrometers: at 30 cm, the eye can no longer separate them and perceives their sum. The typical primaries of a screen have dominant wavelengths of about 465 nm (blue), 530 nm (green) and 620 nm (red). A school-grade colored filter transmits 30 to 60 % of the light within its band, and less than 5 % elsewhere: two stacked filters therefore transmit at best a few percent.

Formula

In additive synthesis, light intensities add up wavelength by wavelength:

I_mix(λ) = I₁(λ) + I₂(λ)

where:

I_mix(λ): intensity of the mixture at wavelength λ (W/m²)
I₁(λ), I₂(λ): intensities of the two superimposed sources (W/m²)

A screen color is written as its triplet, each component being encoded from 0 to 255:

Color = (R, G, B)

White = (255, 255, 255) and Black = (0, 0, 0)

In subtractive synthesis, the transmittances of the filters crossed in succession multiply:

T_total(λ) = T₁(λ) × T₂(λ)

where:

T_total(λ): fraction of light transmitted by the assembly, between 0 and 1
T₁(λ), T₂(λ): transmittances of each filter at wavelength λ

It is this difference - a sum on one side, a product on the other - that explains why one brightens and the other darkens.

Application examples

A white pixel on a smartphone screen is obtained by lighting the three red, green and blue sub-pixels at full power: (255, 255, 255)
A three-color video projector superimposes three red, green and blue images on the screen: this is additive synthesis
An inkjet printer deposits cyan, magenta and yellow inks, each of which absorbs part of the spectrum: this is subtractive synthesis
Stacking a cyan filter and a yellow filter in front of a white lamp gives green: cyan absorbs red, yellow absorbs blue, only green remains
Two blue and yellow theater spotlights lighting the same area give a pale white, whereas the same hues in gouache give green
A screen displaying (255, 128, 0) produces orange: red at full power, green at half intensity, no blue

FAQ

Q: Why do blue and yellow give green in painting but white in light? A: They are two different mechanisms. In painting, each pigment removes wavelengths: blue absorbs red, yellow absorbs blue, only green survives both. In light, spectra are added: yellow contains red and green, to which blue is added, which reconstitutes the three primaries and therefore white.

Q: Aren’t the primary colors red, blue and yellow? A: No, that is an old classroom simplification. The primaries of subtractive synthesis are cyan, magenta and yellow, and those of additive synthesis are red, green and blue. The red-blue-yellow trio cannot cover light, saturated hues, as printer cartridges confirm.

Q: Why do printers have a black cartridge if cyan, magenta and yellow are enough in theory? A: Because in practice the inks are not perfect: their absorption bands do not complement each other exactly. Mixing the three gives a dark brown rather than a true black, costs three times as much ink and soaks the paper.

Q: Is yellow a primary or a secondary color? A: Both, depending on the synthesis. It is secondary in additive synthesis, where it results from red plus green. It is primary in subtractive synthesis, where it serves as a base together with cyan and magenta.

Q: Why don’t we see the red, green and blue dots of the screen? A: The sub-pixels measure a few tens of micrometers, below the resolving power of the eye at normal reading distance. The brain receives their added lights and perceives a single color. A magnifying glass or a macro photo makes them visible.

RGB Additive Synthesis - Colorimeter - Visible Spectrum - Absorbance