In the first two parts of this mini-series on understanding the color theory in your photography, we went over what type of color theory you need to understand (part one) and the science behind that color theory (part two). Now, in the third part of this series, we’ll discuss the difference between RGB and CMYK primaries, why there’s a fourth “primary” in CMYK, and why we even need to have two sets of primaries.
As a quick refresher, look at the above color wheels and remember that in photography (unlike more traditional art forms) we don’t use the RYB color system. Instead, we use RGB, which is a color theory that is based on the biology of the human eye and uses Red, Green, and Blue as primary colors. This is how both you and your camera see the world, and how your computer screen reproduces colors. CMYK, on the other hand, is essentially the inverse of RGB, and uses the secondary colors of Cyan, Magenta, and Yellow as primaries, making Red, Green, and Blue secondaries.
The question is, how can you simply invert a color wheel and get the same results? Well, it’s sort of like how math works the same way with negative numbers as positive numbers. Cyan can be thought of (in RGB) as the presence of Blue and Green, or it can think of (in CMYK) as the lack of Red. Let’s go into that a bit more.
On your screen, the color Cyan is made by lighting roughly equal amounts of Blue and Green within each Cyan pixel you see. If you’re thinking in RGB, this makes Cyan a secondary color because it takes two primaries to make it. This is, as it turns out, how your camera and computer think of Cyan. This RGB primary system is called additive because it is based on adding light to make colors. The base color (that which has no primaries in it) is Black in and additive system, which also means that the center of the color wheel is white because it includes the presence of all three primary colors. Think back to part two, when we learned about the relationship between the cones in our eyes and the primary RGB colors. With all three primaries active, all of our cones are stimulated and we see bright, white light.
If RGB is considered additive, then CMYK is considered (you guessed it) subtractive. This means that the base color is white (stimulating all the cones), and by subtracting the stimulation of one type of cone you get a primary color. If you take all the Red out of white light, you see Cyan, which makes Cyan a subtractive primary color. If your remove all the Green you get Magenta, and if you remove all the Blue you get Yellow. And, to complete the inversion of RGB, if you combine all subtractive primary colors (and therefore remove each additive primary color) you get Black, as seen in the center of the CMYK color wheel.
So which set of primaries is “more correct?” Well, that depends on what you’re using them for. Which primaries you choose is based on the selection of your base color, either White or Black. If your base color is Black, then the primaries you add to it should be RGB. If your base color is White, then the primaries you add should be CMYK. Here in lies the reason behind the presence of RGB and CMYK profiles in Photoshop and other programs.
Your computer screen’s base color is Black, just think of what it looks like when it’s off. The screen then adds the wavelengths of Red, Green, or Blue to each pixel in different combinations to stimulate your eye differently and therefore reproduce colors using the RGB primaries. Paper, on the other hand, has a White base color, just think of a blank piece of paper. This is why CMYK was invented, for physical printing onto paper. Think about gradually adding Cyan to a white piece of paper. It would go from reflecting all light (and therefore being White) to reflecting all light minus a small amount of Red (and therefore being a light-Cyan), to reflecting all light except for Red (and therefore being completely Cyan). That is subtractive color.
Hopefully, this is all starting to make sense, but you’ve likely been wondering what the K is for in CMYK. Well, this is where everything gets, unfortunately, way more complicated, but we won’t go into all the details here. Suffice it to say that the K stands for Black (why they didn’t just use B, I do not know). In short, the ink pigments used to print on paper are not perfect representations of the subtractive CMY system. It’s extremely difficult (or possibly impossible) to make a chemical that absorbs all/most wavelengths of light the stimulate a particular eye cone, so some small amount of the supposedly absorbed light always get’s reflected back anyway. This means that if you try to print black using only Cyan, Magenta, and Yellow ink, you won’t get a true black because each ink will still reflect a small amount of the light it was supposed to absorb entirely. To make up for this fact (and to make printers more efficient when printing Black), printers also have Black ink, which is added anywhere that needs that extra bump of light absorption. It is this difference between the theoretical, ideal CMY subtractive system and the real, ink-based CMYK system that makes a color theory in photography so complicated.
If you want to print exactly what you see on your computer screen, you’re going to need to invent your own, magical inks. There is physically no way to print all of the colors you can see on your screen because the chemistry of ink is imperfect. In theory, if there was an ink system that exactly replicated CMY subtractive color (without needing the K), then you could print the exact colors on your screen. But as it turns out, the colors a printer can produce don’t overlap completely with the colors a screen can produce, so some of what you see on your screen can’t be printed and some of what you see in a print can’t be put onto a screen. Many photo printers partially make up for this limitation by having more than four inks, with some having upwards of 15. Each ink essentially absorbs a different section of the visible color spectrum, which makes for more accurate and vibrant coloring.
In case you haven’t noticed, there are a large number of different color profiles in Photoshop which essentially use different primaries, some RGB based and others CMYK based. Knowing which of these profiles to use for which application is actually really important in producing the colors you’re after. In part four of this series, we’ll go over these different color profiles and explain which ones to use in which situations.