I am not a scientist, but I have found it useful to learn a little about the way light and human eyes work in order to understand what color even is. It’s a surprisingly slippery concept! What even is color? What does it mean for an object to “be” a color? What are the true primary colors, and why? Is the color blue I see the same as the color blue you see? Let’s find out! 

Notes:

• I’m not a scientist, which is a blessing and a curse: I can explain like you’re five, because I am mentally the same age, but I’m also apt to get something wrong. Please correct me if you see that I do. 
• I looked a lot of this up on Wikipedia based on a series of personal “why?” rabbitholes, and I’ve included some diagrams from those sources (credited as best I can) rather than making my own illustrations due to the precise nature of these concepts. 

Light

Without light, there is no color. So let’s start by talking about what light is.

Light is a form of electromagnetic radiation (EMR). That means it’s a form of energy with an electric and a magnetic component. The definition of light is electromagnetic radiation that can be seen by the human eye. This is also called the visible spectrum. Aside from light, there are also other forms of electromagnetic radiation that cannot be seen by the naked eye, such as gamma rays, X rays, ultraviolet, infrared, microwave, and radio waves. 

Measuring EMR

A distinctive property of electromagnetic radiation is that it travels in waves. There are multiple ways to measure these waves:

• Space: Wavelengths are the physical length of the wave from crest to crest. The visible spectrum contains wavelengths between 380 and 750 nanometers (a nanometer is one billionth of a meter).
• Time: Frequency is the number of wave cycles that pass by over a given time frame. The visible spectrum contains frequencies between 430 to 750 terahertz (a terahertz is one trillion cycles per second).
• Energy: Kinetic energy measured in electron volts. I don’t really understand this one. The visible spectrum contains energy between 1.63 – 3.26 electron volts.

For this post I will tend to use “wavelength” terminology, which is most common when talking about light.

The visible spectrum

Let’s take a closer look at the visible spectrum and its wavelengths.

Some interesting things to note about the scientific diagram of the visible spectrum compared to the color wheels we usually see in art instruction:  

• It is a line, not a wheel. 
• Different color ranges of wavelength take up different amounts of space. It’s not even.
• Violet as at one end with the shortest wavelengths.
• Red is at the other end with the longest wavelengths.
• There is no overlap between red and violet. Magenta isn’t on here. In fact, magenta is considered an “aspectral” color, meaning it is not associated with a particular wavelength of EMR, but made up by our brains to explain a particular experience. More on that later!

Why do objects have different colors?

When light hits an object, that object absorbs some of the light and reflects some of it back. For example, a red apple appears red because it reflects light in the ~550ish nm wavelength range and absorbs other wavelengths within the visible spectrum. The broader the spectrum of light we see reflected (the more wavelengths), the brighter and lighter-colored the object appears to us. 

Since reflected light is necessary for color, the amount of light shining on the object also plays an important role. Whether an object is able to reflect light in a particular spectrum depends on there being light to reflect. Shadows always look darker than lit sides of objects. In a dark room, all objects look dark. It could be truthfully said that the same apple that “is red” when it is well-lit is not red when it is dark. Color only exists in light. 

Color also only exists in our minds, as we’ll see. 

Human color vision

It’s important to note that there is a wide range in variation in what is normal for humans, including blindness and colorblindness. For the purposes of this post, I’ll mostly talk about human color vision. I keep specifying “human” not because I think color vision is intrinsic to humanity, but because different animals’ color vision works differently. 

Trichromatic color vision

The eye contains special cells designed for perceiving light, called photoreceptors. Humans have two kinds, named rods and cones, based on their shape. Rods are responsible for low-light vision and can’t see color at all. Cones are responsible for color vision and fine detail. 

We are said to have trichromatic color vision because our cones come in three types. While all cones respond to all wavelengths of light, one type of cone is dominant in different wavelength ranges:

• L for Long: red dominant
• M for Medium: green dominant
• S for Short: blue dominant

Below is a diagram of the spectrum of light from our sun, with wavelength increasing from left to right and bottom to top. The fact that it seems to visually resolve into red, green, and blue sections demonstrates our trichromatic visual system. (To me they look like red-orange, green, and blue-violet, but we’ll continue with the red/green/blue terminology for simplicity.) 

Opponent process theory

Opponent process theory says that the brain interprets colors as a series of opposites. You could think of them as sliders from fully one color to fully its opposite or complement.

• Red vs. cyan
• Green vs. magenta
• Blue vs. yellow

An immediate and obvious piece of evidence for this theory is the phenomenon of afterimages, like this American flag example.

These opposites make it quite reasonable to represent color as a wheel with the opposite, or complementary color, across the wheel from each other.

Color mixing

What happens when you mix colors? It depends on whether you’re mixing light or pigment. The difference is whether the thing you are mixing emits its own light, or reflects light.

Light works by additive color mixing. When you add two beams of light together, the mix contains the combined spectra of both beams – a wider range of wavelengths than either individual beam. Adding more lights together creates brighter colors. If you add together every color of light, it makes white.

Pigment works by subtractive color mixing. When you add two paints together, the mix absorbs the combined wavelengths absorbed by either, and reflects only the overlap of wavelengths that are reflected by both. A mix reflects a narrower spectrum of light than the components. Adding more pigments together creates darker/duller colors. if you add together every color of pigment, it makes black.

Summary	Additive Color Mixing	Subtractive Color Mixing
Works for…	Light	Pigment
Examples:	Computer screen, LED lights	Paint, dye, printer ink
Primary colors:	Red, Green, Blue	Cyan, Magenta, Yellow
Mix all primary colors to get…	White	Black
Mixes are…	Brighter than components	Duller than components

Primary colors

For humans with color vision, the primary colors for additive color mixing (light) are red, green, and blue. That’s why this type of color mixing is also called RGB. Each of these color ranges is best seen by one of our three cone cells. Our definition of these colors, and light itself, is based on human eyes and brains; it would be different for another animal. 

The primary colors for subtractive color mixing (paint) are cyan, magenta, and yellow. These are the visual opposites of the primary colors of light, according to opponent process theory.

Actionable insights about color mixing

What are the actionable insights you can take away from this to your art?

• Paint uses subtractive color mixing, for which the primary colors are cyan, magenta, yellow.
• The more paints you mix together, the closer you get to neutral gray/black (low chroma).
• You cannot make a color brighter (higher chroma) by mixing. Every mix is lower-chroma than the component parts.
• You can maintain a high-chroma mix by mixing colors that are similar to each other and have a lot of overlap.
• You can create a very low-chroma (neutral gray) mix using just two bright paints if they are opposites (complementary colors.)

Conclusion

I hope that this information was accurate (more or less) and helpful. Some people are overwhelmed by sciencey stuff, but I think that some level of understanding can really help to get your mind around why color mixing works the way it does, and what we are even doing here when we paint in color. 

If you would like to know more, I would like to take a moment to recommend some resources: 

• The Secret Lives of Color by Kassia St. Clair contains a brief intro about additive and subtractive color mixing that explains this so concisely and blew my mind.
• Color and Light by James Gurney contains a lot of good information for artists about color, both sciencey and arty.
• On the extremely weedsy side, set aside an afternoon to dig into The Dimension of Color by David Briggs.