Primary and secondary colors are things we learned about in grade school. Many of us can still remember the lesson on red, yellow and blue. We learned that they are the primary colors. But wait. Yellow isn’t a primary color to a graphic artist designing CGI images. What gives?
The relationships between primary, secondary and tertiary colors are important to the concept of color mixing. The science behind it accounts for both additive and subtractive color mixing. Understanding these two types of mixing explains why artists working with traditional media understand red, yellow and blue (RYB) as primary colors, while digital designers work with red, green and blue (RGB). A third color model used specifically for printing is called CMYK.
Additive and subtractive color mixing are so named because of the way different colors are achieved. As you might have guessed, additive color mixing involves adding color. Subtractive mixing involves taking color away. This post will explain it all in detail.
Additive Color Mixing in the Digital World
We start with additive color mixing because it is the easier of the two to understand. Additive mixing is the domain of digital graphics, electronics, etc. Primary colors in this realm are red, green and blue. When you see a reference to RGB in photo editing software, for example, it refers to those three primary colors.
Additive mixing is accomplished by adding different colors from the spectrum of visible light. The easiest way to understand this is to think of a television set or a laptop computer display. In both cases, the screen is black when the display is off. Why is it black?
Black is actually not a color. A TV screen is black when it’s off because no light is emanating from it. Where there is no light, there is no color. Moreover, the surface of the screen absorbs all of the ambient light hitting it from all directions. Where there is no reflection of light, color is equally absent.
Turning the screen blue would be a simple matter of switching on the power and energizing it with blue light. The same goes for turning it red or green. That’s exactly how televisions and laptop screens work. The electronics inside manipulate liquid crystals to project certain colors. The colors combine to constitute the images we see.
Mixing the Colors
If the world we lived in consisted only of separate instances of red, green and blue, that’s all you would see on your television screen. But we know that’s not the case. The human eye is capable of seeing millions of colors. To replicate what we can see naturally on an LCD screen, internal electronics have to mix colors accordingly.
In the RGB world, mixing red and green produces yellow. Red and green are primary colors; yellow is a secondary color. Mixing equal parts of two primary colors produces a secondary color. Mixing equal parts of one primary color and one secondary color produces a tertiary color.
An electronic display produces those millions of colors your eyes see by constantly combining primary, secondary and tertiary colors. It’s all done by adding light to the display. Likewise, that split-second of black you notice as a TV show switches to commercial is achieved by removing all of the light from the screen.
Subtractive Color Mixing in the Art World
Subtractive color mixing is best understood by looking at it from the perspective of art. This takes us back to third grade art class. Remember being given a piece of white paper and a paint set? You were encouraged to paint your parents a beautiful picture with poster paints or watercolors. You had no idea you were practicing subtractive color mixing, did you?
Remember that additive color mixing is accomplished by adding light. Subtractive color mixing is just the opposite. We mix colors by subtracting light from the equation. That white piece of paper from art class should make this fairly easy to understand.
Just like black, white is not a color. It is the presence of all of the colors in the visible light spectrum. Your eye recognizes a piece of paper as white because all of the ambient light hitting it reflects off it into your eye. Your eye sees every color in the spectrum combined and perceives it as white.
Mixing the Colors
Suppose you wanted to paint a red dot on a white piece of paper. How and why would your eyes see red? Your eyes would perceive red because of the pigment in the paint. That pigment absorbs all of the colors in the spectrum except red. The red light reflects off the paint and is picked up by your eyes.
Now, remember that you just covered a portion of the paper with the paint. You blocked out that section so that no light could reflect off of it. The pigment now absorbs everything but red. It is subtracting all of the other colors from the spectrum. This is subtractive color mixing.
We learned in art class that combining blue and yellow paint using the RYB color model produces green. It’s the same story. The pigments in both paints, when combined, block out all of the colors in the spectrum except green. Blocking those colors subtracts them from the light that reflects off the surface and reaches your eyes.
Why the Difference Between Additive and Subtractive Color Mixing Exists
Hopefully, you are not totally confused by all of this. Perhaps you are wondering why there is a difference between additive and subtractive color mixing. It has to do with how the eye perceives color and light.
Light emanating from a computer display goes directly to your eye. That light activates photo receptors, or cones, in your eye. It turns out that the human eye perceives red, green and blue naturally. This act of nature dictates that the three primary colors for light directly penetrating the eye are red, green and blue.
We add color in the additive model because we are starting with black. Again, remember that a deactivated computer screen looks black for lack of light emanating from it. We start with black and add color to generate something the eye can see.
A piece of paper is not black because light reflects off it. That’s true whether the paper is white, cream or some other color. Because we aren’t starting with black, we cannot create colors by adding light. Instead, we have to subtract. The fact that our primary colors are now different is due to the way the eyes perceive reflected light.
Reflected light is not pure, unaltered light. It is altered by whatever surface it is reflecting off of. This suggests that the photoreceptors in the eyes don’t perceive reflected light quite the same way they perceive direct light. Without attempting to explain the biology, this is why the third primary color in reflected light is yellow rather than green. It also explains why blue and yellow create green in subtractive color mixing while red and green create yellow in additive mixing.
Additive and subtractive color mixing can be difficult to understand. It boils down to the source of light. If light is reaching your eye directly from the source, color is created through additive color mixing. We start with black and add color to create images. If the source is reflective light, color is achieved through subtractive mixing. We start with the color of the reflecting surface and subtract light from there.
Regardless of the science behind it, additive and subtractive color mixing add beauty to our lives. Thanks to the eye’s ability to perceive color and the brain’s ability to decipher it, we can see millions of colors in the natural world. And that should make us all appreciate our eyesight. After all, what would life be like without color?