It’s a common misconception that mixing the colors red and blue will make purple. This seems intuitive, as the three primary colors are red, blue, and yellow. However, when light mixes additively to create color, red and blue actually combine to make magenta, which is a vivid reddish purple. This difference occurs because of the way our eyes perceive color.
How Light Creates Color
White light is made up of all the colors of the visible spectrum. When light hits an object, some colors are absorbed while others are reflected. The reflected colors are what our eyes see as the object’s color. For example, a tomato appears red because it absorbs all colors except red, which is reflected back.
Our eyes have three types of color receptors called cone cells. Red, green, and blue cone cells respond to different wavelengths of light. When red and green light enters our eyes in equal amounts, our brain interprets this as yellow. When all three cone cells are stimulated evenly, we see white. This is how mixing different colored lights creates new colors through an additive process.
Why Red and Blue Make Magenta
When red and blue light mix together, they stimulate the red and blue color receptors in our eyes. There is no green light to activate the green cones. Since the red and blue cones are activated equally, our brain interprets this mixture as magenta.
Magenta is not part of the visible color spectrum – it is an extra-spectral color that our eyes and brain create when seeing a particular combination of wavelengths. Other extra-spectral colors include white and brown.
So while red and blue are primary pigment colors, when mixing light they combine to make magenta, not purple. This distinction is important in fields like photography, lighting, monitors, and printing where additive color mixing is used.
Color Mixing Models
There are two main models for mixing colors:
Additive: Colors combine by adding light. Used for mixing colored lights, TV/computer screens, photography. The primary colors are red, green, and blue. Combining all three makes white.
Subtractive: Colors combine by absorbing certain wavelengths. Used for mixing paints, dyes, inks. The primary colors are cyan, magenta, and yellow. Combining all three makes black.
| Additive (RGB) | Subtractive (CMYK) |
|---|---|
| Red | Cyan |
| Green | Magenta |
| Blue | Yellow |
| Mix all = White | Mix all = Black |
As shown in the table, the primary colors are different for additive and subtractive mixing. This explains why combining red and blue pigments makes purple, while red and blue light makes magenta.
Why Red and Blue Pigments Make Purple
When working with pigments, dyes, or inks, colors mix through subtraction. These materials absorb certain wavelengths of light and reflect the rest.
Red pigment absorbs green and blue light, reflecting just red. Blue pigment absorbs red and green, leaving blue. When red and blue pigments are combined, both green and red light are absorbed, leaving just blue to be reflected. Our eyes perceive this mixture as purple.
So with pigments, red and blue make purple because green light is subtracted out. The reflected blue mixed with the reflected red appears purple to our vision. This subtractive mixing of pigments follows different rules than the additive mixing of light.
Other Examples of Additive vs Subtractive Mixing
Here are some other examples showing the difference between additive and subtractive color mixing:
Red + Yellow Light = Orange
Mixing red and yellow light activates the red and green color receptors in our eyes equally. Our brain interprets this as orange.
Red + Yellow Paint = Reddish orange
Combining red and yellow paints means both blue and green light are absorbed, leaving mostly red and some yellow light reflected. We perceive this as a reddish orange.
Green + Blue Light = Cyan
Green and blue light together stimulates the green and blue color receptors, which is seen as cyan.
Green + Blue Paint = Dark green
Green and blue paints absorb red light, leaving mostly green light to be reflected, creating a darker green color.
So while the same color combinations can look very different depending on the mixing model, they follow predictable rules. Understanding additive and subtractive color theory helps explain mixtures like why red and blue make magenta light but purple paint.
Practical Applications
The distinction between additive and subtractive color mixing has many practical applications:
Photography: Red and blue make magenta, an important color in photo developing. Filters and lighting alter color mixing.
Computer/TV Screens: These displays mix light additively using RGB pixels. Different combinations make all screen colors.
Printing: Uses CMYK inks as subtractive primaries. Combinations make a full color gamut.
Painting: Pigments mix subtractively. Artists must understand these color interactions.
Lighting: Stage/event lighting uses red, green and blue gels, mixed additively.
Clothing/Dyes: Fabric and clothes use dye/pigment subtraction. Colors combine differently.
In all these fields, professionals must understand both additive and subtractive color mixing to create accurate colors. While counterintuitive at first, the reasons why red and blue light mixes to magenta vs. purple paint follow predictable color science.
Color Perception
Our perception of color also helps explain why red and blue don’t make purple. Red, green and blue are psychological primary colors because of how human vision works. We have receptors keyed to those three colors.
But there are no pure purple receptors – our eyes don’t actually see purple light. We perceive purple when our red and blue receptors are stimulated and green is absent. This demonstrates the subjective nature of color.
For example, purple and violet seem similar but have different mixes of red and blue light. Our vision and brain constructs these colors out of the available inputs. The same principles apply to extra-spectral colors like magenta.
So red and blue making magenta instead of purple has to do with physics, color theory and psychology. While not intuitive, the reasons follow consistent rules about how we see and process color.
Conclusion
In summary, red and blue combine differently depending on whether colors mix additively with light or subtractively with pigments. Red and blue light make magenta, activating red and blue color receptors in our eyes. Red and blue paints make purple by absorbing green light and reflecting both red and blue.
Understanding these color interactions has many applications in photography, design, printing and more. While our brain can play tricks on us, the reasons red and blue don’t make purple stem from consistent, predictable color science. Color theory explains how the same two colors can mix so differently, providing valuable insights for working with color across fields.
