The Difference Between Primary Colors of Light and Pigment
In our daily study and life, we often hear that the three primary colors of pigment are "red, yellow, and blue," while the three primary colors of light are "red, green, and blue." Have you ever wondered why, despite both being called "primary colors," they are different? What distinguishes them?
The so-called pigment primary colors (CMYK) — often simplified as "red, yellow, blue" — should more accurately be referred to as Magenta, Yellow, and Cyan. Meanwhile, the light primary colors (RGB) are Red, Green, and Blue (which correspond to the visual sensation of bright red, medium green, and cyan-blue in pigments).
The most fundamental distinction between pigment and light primaries lies in their mixing principles. Color theory divides color mixing into additive mixing and subtractive mixing.
Additive vs. Subtractive Mixing
Additive mixing (also called light mixing) involves combining different colored lights. When lights mix, the brightness increases. This principle is commonly used in screen displays, such as monitors, TVs, and projectors.
Subtractive mixing is the most common method in painting and printing. It involves mixing pigments or dyes, where each substance absorbs (subtracts) specific wavelengths of light. Mixed colors become darker and less pure.
Primary Colors of Pigment (CMYK)
British chemist George Field and other researchers determined that Magenta, Yellow, and Cyan are the optimal primary colors for pigments and other non-luminous materials. These three can be mixed to produce a wide variety of colors, though they cannot create a true black — only a deep gray. Therefore, in color printing, besides the three pigment primaries, a black (Key) ink is added to achieve deep, rich tones.
By definition, primary colors should be able to mix most other colors, while other colors cannot mix to recreate the primaries. For example, art students learn that magenta mixed with yellow produces red, but red cannot be separated back into pure magenta. Cyan plus magenta yields blue, but blue plus green yields a dull cyan, not a vibrant one.
Mixing yellow, magenta, and cyan pigments can create purer and brighter secondary colors: cyan plus yellow produces a vibrant green, purer than the green from blue plus yellow; magenta plus cyan creates a vivid purple, unlike the murky purple from red plus blue.
In practical applications — such as color printing ink formulation, color photography, and color printer design — Yellow, Magenta, and Cyan serve as the three primary colors. Color printing uses these three inks plus black. A four-color printing press is a classic example. In color photography, three emulsion layers are used: the bottom layer is sensitive to yellow, the middle to magenta, and the top to cyan. Most color inkjet printers also use yellow, magenta, cyan, and black cartridges to print color images.
Primary Colors of Light (RGB)
Based on research by English physicist Thomas Young and German physicist Hermann von Helmholtz, the primary colors of light were determined to be Red, Green, and Blue. Mixed in certain proportions, they can produce various colors of light. Color TV and monitor screens are composed of tiny red, green, and blue light-emitting dots.
You may recall from school physics experiments: white light passing through a prism disperses into a spectrum of colors ranging from red, orange, yellow, green, cyan, blue, to violet — the visible spectrum. The human eye is most sensitive to red, green, and blue. Our eyes function like a three-color receiver system. Most colors can be synthesized by combining red, green, and blue light in different proportions. Similarly, most single-color lights can be decomposed into these three. This is the most basic principle of colorimetry — the Trichromatic Theory.
Color TV screens are coated with three different phosphors. When electron beams strike them, one emits red light, one green, and one blue. During manufacturing, these phosphor dots are arranged in alternating patterns across the screen. Any three adjacent dots will include one red, one green, and one blue. Each dot is only pinhead-sized, invisible to the naked eye without magnification. Because they are tiny and tightly packed, when they glow, our eyes cannot distinguish the light from individual dots, instead perceiving a blended color.
Additive and Subtractive Mixing Principles
In light-emitting objects, we see additive mixing: Red + Green = Yellow; Green + Blue = Cyan; Red + Blue = Magenta; Red + Green + Blue = White. Yellow, cyan, and magenta are each formed by mixing two primary lights, so they are called additive secondary colors. Furthermore, in light: Red + Cyan = White; Green + Magenta = White; Blue + Yellow = White (this is why LED backlights in LCD displays often use blue LEDs combined with yellow phosphors to produce white light). Thus, cyan, yellow, and magenta are also the complementary colors of red, blue, and green, respectively.
Under white light illumination, the colors we see from pigments and non-luminous objects exhibit subtractive mixing: White - Red = Cyan; White - Green = Magenta; White - Blue = Yellow. Additionally, if we mix cyan and yellow pigments, under white light, since the pigments absorb red and blue light and reflect green, we represent pigment mixing as: Pigment (Yellow + Cyan) = White - Red - Blue = Green; Pigment (Magenta + Cyan) = White - Red - Green = Blue; Pigment (Yellow + Magenta) = White - Green - Blue = Red. The above describes subtractive mixing, where different colors are formed by absorbing different proportions of the three primary colors. In pigment primaries, red, green, and blue are called subtractive secondary colors or pigment secondary colors. In subtractive mixing: (Cyan + Yellow + Magenta) = White - Red - Blue - Green = Black.
In summary, pigment primary colors generally refer to the basic colors represented by the subtractive mixing of the CMYK model, belonging to the printing mode, widely used in painting and printing. Light primary colors generally refer to the basic colors represented by the additive mixing of the RGB model, extensively used in lighting and color displays.
Essentially, the CMYK model is not fundamentally different from RGB; they simply produce colors in different ways. Displays use the RGB model because they emit light by electron beams striking phosphors on the screen. When there is no light, the screen is black; when light is at maximum, it appears white. Printers, however, use inks that do not emit light themselves. They can only reflect other light waves by absorbing specific ones, hence requiring the subtractive method.