Why Red, Green, and Blue Are Primary Colors of Light, Not Red, Green, and Yellow

There are two sets of primary colors: one for pigments and another for light. Understanding these differences can be surprising and enlightening. In this article, we'll explore why red, green, and blue are considered primary colors of light, not red, green, and yellow as one might initially assume.

The Primaries for Pigments

When working with pigments such as paints or printing inks, the primary colors are red, yellow, and blue. These are known as subtractive primaries. This term comes from the fact that mixing pigments removes wavelengths from the light, making the colors more intense or darker.

When you mix all three primary pigments, you get a dark or black color. This happens because pigments absorb certain wavelengths and reflect others. In an ideal scenario, these three pigments would absorb all wavelengths of light, resulting in pure black. However, in reality, no pigments are perfect, and they often reflect some residual wavelengths, making them appear as various shades of gray.

The Primaries for Light

Unlike pigments, the primary colors of light are red, green, and blue. This might seem counterintuitive at first, but there's a scientific explanation for why these colors are primary for light. When mixing light, red, green, and blue can create a wide range of colors, including the full spectrum of visible light.

The visible spectrum can be divided into three main sections: red (long wavelengths), green (medium wavelengths), and blue (short wavelengths). These three colors, when combined in various proportions, can produce a vast array of colors, from intense royal blue to strong purplish blue, and everything in between.

Human Color Vision and Evolution

Our understanding of primary colors is closely tied to how human eyes perceive color. Human eyes have three different types of cone cells, each sensitive to a specific range of wavelengths. These corresponding colors are red, green, and blue. The combination of signals from these cone cells allows us to perceive a wide range of colors.

Interestingly, if our species had different types of cone cells sensitive to other wavelengths, those colors would be considered primary instead of red, green, and blue. This is a fascinating example of how biology shapes our perception and interaction with the world around us.

Creating Intermediate Colors

When it comes to light, intermediate colors can be created by mixing light of different wavelengths. The key point here is that these intermediate colors are created based on the additive properties of light. You can't achieve a blue color by combining red and green light, as it would simply result in a color that has a mixed appearance, not a pure blue.

Red, green, and blue are the primary colors because they can be used to create a broad spectrum of light. There is no combination of red, green, and yellow that can produce a true blue. This is because blue is shorter in wavelength than both red and green, and the way light is perceived by human eyes is based on these specific primary colors.