The Primary Colors Aren't Red Yellow and Blue

The chart below represents visible light as a prism would create it. The numbers are the frequency of that light. Just like sound, light has a frequency. As you can see, the visible spectrum represents about 1 octave (one doubling of frequency).

The chart below is from Wikipedia. As you can see, there's both the wavelength and frequency, which are inversely proportional. One represents the length of the wave, and the other represents how often they occur. I'm only going to talk about frequency because coming from audio, that's a language I'm comfortable and familiar with, though this is probably referred to less often than wavelength. You can see the graph they give focuses on wavelength. Of course, it doesn't help that the numbers of the two ranges overlap a great deal.

Just by looking at these graphs, it would seem obvious that the primary colors are Red, Green, and Blue, not Red, Yellow, and Blue.

How Does The Eye Detect Color?

When we see something as being blue, that object absorbs every color except blue, which it reflects in to our eyes. So a white light, which has every color in the spectrum, hits a blue object. That object absorbs colors, but reflects blue. But how does our eye detect this, and how does the brain interpret it?

From Richard A. Muller's Physics Textbook:

On the back surface of the eye are kinds of sensors, which we will refer to as the rods, the blue cones, the red cones, and the green cones. The green cones are not green in color; we call them that because they are sensitive to light near the green region of the spectrum. Look at the plot below. The green curve shows the wavelengths over which the green cones respond to light. Note that they respond most strongly in the green region, but they also respond if the light is orange, or even blue. The red cones respond most to yellow light, but they respond a little bit to red light (which the green cones don't detect).

Notice that green light is detected by all three cones: most strongly by the green cones, a bit weaker by the red cones, and weakest of all by the blue cones. When the brain gets this combination of signals, it calls the color as green. If it receives a strong red, a weaker green, and no blue at all, then it tells you the color is yellow. (Can you see that in the diagram?)

Blue. Green. Red. These are the only colors the eye really sees. Everything else is an extrapolation created by the eye.

RGB and CMYK

As you can see, the eye actually only detects 3 colors: Red, Green, and Blue. Depending on the strength of the signal of each it detects, the brain decodes it as one of the colors of the rainbow. You may have heard of RGB before, all televisions and computer monitors are based on these colors. This is the additive color system, and when you combine all of them, you get white.

That is, you start with a black surface (like a computer monitor), and add a red light. Then a green light, and then a yellow light. When mixed together, perhaps due to the way the eye perceives color, it becomes white.

The other system, used for print, is CMYK. Cyan, Magenta, Yellow, and Black (K for Black). This is the system used for almost all printing. This is a subtractive system. Starting with a white surface (like a piece of paper) and adding different amounts of Cyan, Magenta and Yellow you can create almost any color.

That is, each time you add a color, it darkens the whole. In theory combining all of them creates black, but in reality, due to sleight imperfections in the inks you end up with brown, so there's an extra black ink cartridge to do blacks. Plus it's probably more economical to use a single black cartridge than 3 color cartridges.

Red, Green, Blue RGB	Cyan, Magenta, Yellow CMYK

You can see that the in-between colors in RGB are brighter than the primaries. This makes sense, because it's an additive system, and when you add two colors together, they get brighter. Yes the colors they create are Cyan, Magenta and Yellow.

In CMYK, you start with these brighter colors, and mix them together to get darker colors.

To get darker colors in RGB, you simply add less light, and to get lighter colors in CMYK, you add less ink (allowing more of the white of the paper come through), or mix in white paint.

Red Yellow Blue

For centuries, Red, Yellow and Blue were considered to be the primary colors. These colors were mixed to create an array of other colors. (Okay, it's not a conspiracy.) A great range of colors can be mixed from Red Yellow and Blue. Yellow can be seen as a shifted green, or Red and Blue as a shifted Magenta and Cyan.

From the Physics Hypertextbook:

The painter's color wheel is an historical artifact that refuses to die. The primary colors are not red, yellow, and blue. Painters and art teachers promote this scheme. It is a convenient way to understand how to mimic one color by mixing red, yellow, and blue. But these colors do not satisfy the definition of primary colors in that they can't reproduce the widest variety of colors when combined.

The below graphic should show you how versatile CMY are when compared to RBY.

Cyan, Magenta, Yellow	Blue, Yellow, Red

I lightened up the Red, Yellow, Blue graph so each color was at 60% opacity (that means it's 40% see through) so you could see a little better what's going on. You can see a purple and orange, but not much of a green. Next to it is a traditional color wheel that you might find on the back of a box of crayons.

Red, Yellow, Blue at 60% opacity.	Traditional Red, Yellow, Blue Color Wheel

Magenta

The odd thing about CMYK is that Magenta isn't in the spectrum of colors.

Cyan can be found in the range of 600 to 620

Yellow can be found in the range of 510 to 530.

But where is Magenta? It almost seems like it would appear if only the spectrum would continue on and wrap around to red again.

From Ask a Scientist:

The color magenta stimulates both the "red cones" and the "blue cones" in your retina. A combination of red light and blue light will do exactly the same thing.

From WFMartin on WetCanvas.com

Most of what the human eye views as "yellow" is actually equal reflectance of red and green light, and not spectral yellow at all (only about 7%). Most of what we view as cyan is equal reflectance of blue and green light, and very little spectral cyan. What we view as the color, magenta, represents equal reflectance of red and blue light, and, since those two colors are at opposite ends of the natural spectrum, is not present in the spectrum at all. Many artists argue this point (that the color magenta is not present in the spectrum), but very few color theorists argue it. The color,magenta, exists in pigments, in inks, in dyes, in color wheels and color models--just not in the natural spectrum. Hard for some folks to comprehend. Why doesn't it exist in the spectrum? Because the two color wavelengths which produce it aren't side by side in the spectrum.

If you look back at the RGB chart on this page, you'll see that in an additive system, Magenta is a combination of Red and Blue. But Red and Blue aren't next to each other in the rainbow. They exist on a line, not in a triangle, and Blue never connects to Red again, though it begins to as the spectrum, begins to reach the octave mark where it would loop back to Red, which is where Violet lives.

But we can see Magenta, even if it doesn't exist as a single frequency of light in the color spectrum. Remember, the eye perceives color as a combination, or lack of the three Additive Primaries.

Magenta is actually the absence of Green light. Magenta isn't a pure color (a single frequency), it's a combination of red and blue. This is why purple is so rare in nature, and it's this rarity that made it "royal." The following graph shows the color spectrum without Green. As you can see the white numbers have become Magenta and they show up against the violet background (which they wouldn't do if they were the same color).

Mixing Colors In Reality

The reason I'm doing all this is that my girlfriend is working on some projects that involve mixing colors. She wanted to know if she needed to buy an array of colors, or if she could just buy a few primary colors and mix them to create other colors. The research I've done shows me that while you can mix Cyan, Magenta and Yellow to get other colors, they won't be as pure as just buying the appropriate color.

In reality, mixing Magenta and Yellow to get Red gave her a color, something like the below picture, but darker. Red, but dull. Not the vibrant red you'd expect knowing that my printer has only Cyan, Magenta, and Yellow to make Red appear on the paper. It's still pretty amazing that you can start with Magenta, add Yellow and end up with Red. Not a pure Red, but Red nonetheless. No amount of white can brighten it up because while the hue is fine, the saturation wasn't right.

The picture below is dithered. 50% of the pixels are cyan, and the other 50% are yellow.

Magenta, Yellow Dither	Magenta, Yellow Dither (Close Up)

From handprint: color mixing theories.

A red paint reflects light only from the "red" end of the spectrum; it stimulates the R cones, but not the G or B. Most blue paints reflect mostly "blue" and some "green" light, stimulating the B and G cones, but not the R. So their mixture does not create a clear purple but a very dull color near black, because the two colors have no reflectance in common: every wavelength reflected by one color is absorbed by the other.

Unfortunately, these tradeoffs also mean that mixtures of two subtractive "primaries" reflect light from all parts of the spectrum. The result is a flatter cone response profile, which creates the perception of a less saturated color mixture — a color closer to gray. This is half the explanation for the saturation costs in paint mixtures — the tendency of paint mixtures to be darker and grayer than the original paints. (The other half is the additive saturation costs that result from the overlap in the R, G and B cone sensitivity curves.)

In other words, if you mix two colors, one of them will absorb some of the light the other reflects, and the result will be more dull, or gray, than a pure version of that color. A pure red will reflect light from just that part of the spectrum, but a red made from mixing two colors together will reflect light from different parts of the spectrum and the brain will put them together to make red. Since what's hitting the eye is a scattered mix of colors, it results in "a flatter cone response profile," and we percieve it as being less vibrant.

Color mixing is a complex art and science, and it takes a while to understand all the complexities. Hopefully this overview is helpful and will point you in the right direction if you're interested in learning more.

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