How Full Color Printing Works

What CMYK Means

CMYK stands for the names of the four primary colors used in full color printing. (That’s right, red, blue, and yellow are not the only possible primary colors, your kindergarten teacher lied.) They are:

• Cyan

• Magenta

• Yellow

• Key (Black)

CMYK was created specifically for printing. The colors are printed as tiny dots. Since our brains are both lazy and smart, rather than pick out every dot, they turn those dots into solid blocks of color. It’s the proportion of each primary color that determines how your brain perceives the overall color.

Why is black called key? Well, in old school printing, in which they used different plates for each color, the key plate had the most detail and was usually the black plate. While the whole K-not-B thing might be confusing now, you’ll be grateful for it when we get around to talking about RGB, because that B stands for blue.

The Color Theory Behind CMYK

CMYK is an example of a color model, which is a way to define color using math. You express CMYK colors in terms of percentages of each color. For example, C90 M10 Y45 K0 is a light mint green that leans more blue than yellow, since it uses 90% of the possible available cyan and 45% of the available yellow. We’ll get to what that actually looks like when it’s printed IRL in a bit.

Importantly, CMYK is a subtractive color model. That means that to get to white, you have to have less of each color. Take our mint green, C90 M10 Y45 K0. If you decreased each primary color by about 10% and changed it to C80 M9 Y40 K0, your green would still lean minty, thanks to the dominance of the cyan, but it would be a lighter version, because the percentage values are lower.

This all makes more sense when you think about it in terms of printing. In printing, you’re adding ink to white paper. Ink absorbs light. We need light to see things, so when something reflects a lot of light, it appears very bright to us, and when it absorbs a lot of light, it appears darker. So the more ink you have – the higher the CMYK percentages – the more light is absorbed and the less light is reflected, so the darker the color looks.

Side note about black ink. When you combine cyan, magenta, and yellow – so all of them at 100% – you get black. So why do we need a separate black ink? Why couldn’t it be CMY or three-color printing?

Well, printing nerds decided that the black created when you mix these inks isn’t pure enough. It’s referred to as “theoretical black,” as opposed to “actual black,” which is a pure black ink. Plus, black ink is useful for darkening the overall impression of the color without changing the essence of the color itself.

Say you wanted to take our mint green and make it darker. If you didn’t have black, you’d have to increase the magenta and yellow, but that creates a different kind of mint – less minty, more pinky/yellow. With CMYK, you can get a darker shade of the same mint green by increasing the percentage of black.

How CMYK is Printed

I said I would explain what “90% of the available cyan” actually looks like and here we go.

CMYK uses a printing technique called halftones. Remember how CMYK is printed as tiny dots? Halftones are used to represent the different percentages of each primary color called for in those dots. It’s very annoying that they’re called halftones, because it implies that they are always half, when in fact, the whole point is that they correspond to the CMYK percentages.

We talked about how more ink on your page (higher CMYK percentages) means more light absorption, which results in darker colors. And vice versa – less ink means more light reflection, which means lighter colors. Halftones create those differences in ink volume in different ways:

Amplitude-modulated (AM) Dots:

The percentages refer to dot size. The dots are all arranged in a grid, and the higher the percentage, the bigger the dot. So if you’re printing our mint green with CMYK values C90 M10 Y45 K0 in AM dots, you’ll have most of a cyan dot, a tiny magenta dot, an almost-half-size yellow dot, and no black dots.

Frequency-modulated (FM) Dots:

The percentages refer to dot frequency. The dots are all the same size, but a higher percentage tells the printer to produce more of them. So let’s say you want to use FM dots to print a picture of a leaf in our mint green C90 M10 Y45 K0. In the mint green areas, the printer is going to cluster a lot of cyan dots, hardly any magenta dots, half as many yellow dots as cyan dots, and no black dots.

AM dots are easier to print and therefore most commonly used, especially for text. However, our clever brains just love to see patterns, including grids. Depending on how the dots fall in your piece, your brain might pick out weird overlays (called moiré patterns) that spoil the effect and stop you seeing all those dots as seamless blocks of color.

Another variable with CMYK printing is that the dots can overlap (overlapping halftones), which creates your new color by combining the inks on the page – like how you would mix red and blue paint to make purple. Or the dots can sit next to each other (juxtaposed halftones) and rely on your brain to perceive them as one block (which is called optical blending.)

Overlapping halftones create a richer color, because you’re essentially mixing a new ink. But they’re more complicated to get right, since you have to account for factors like ink viscosity and the order in which colors are printed.

How CMYK is Different From RGB and Spot Colors

CMYK isn’t the only way to classify colors. You’re about to be happy about that K!

RGB

Like CMYK, RGB is a color model, meaning it uses mathematical calculations to define colors. RGB is sort of like CMYK’s twin. It uses the same logic, in that it combines primary colors – red, green, and blue – to create a wide range of other colors. But RGB is used for screens, not printing. It’s made up of dots formed of tiny light beams, not ink. And because it uses light, it’s an additive color model: to get closer to white, you add colors.

Instead of using percentages like CMYK, RGB colors are defined by assigning each of the three primary colors a value between 0 and 255. So R0 G255 B0 is pure green. R255 G255 B255 is white, and R0 G0 B0 is black. You might be familiar with RGB as expressed in hex codes, which are most commonly used to translate RGB into code computers can read. That’s important, given that RGB is used in device screens.

If the idea that increasing the intensity of a color can make the overall color lighter sounds weird, imagine three flashlights that each have a red, green, or blue bulb. If you shone the light at a wall so the beams overlapped, the circle at the center would look white. If you turned off the green flashlight, the light would get dimmer – and the overlap would now look magenta.

The fact that circle is white — all the colors of visible light combined — is how we know that red, green, and blue are the primary colors in light. Scientists think the reason it’s red, green, and blue light specifically is because these particular wavelengths of light interact with the color-detecting cells in our eyes.

Here’s where we get to the fun relationship between CMY and RGB. Cyan, magenta, and yellow are secondary colors when you’re using light – the colors made by mixing two primary colors. For example, when you mix green and blue light, you get cyan. This same logic is reversed in subtractive color models: The reason cyan ink looks cyan is because it absorbs red light and reflects green and blue – which combine to make cyan.

Spot Colors

Spot colors are also used for printing. But whereas CMYK colors are created by the printer combining those four primary colors in different proportions, spot colors are printed in premixed ink.

In other words, if you want that mint green, you can’t just hit “Print” and let the printer figure it out. You’ve got to find the exact shade of mint green ink that you want. Because of this, spot colors are mainly used in projects that require only a few colors – typically in pad printing or screen printing – or where you want to be extremely precise.

Possibly the most famous spot colors are Pantone colors. Color nerds Pantone created a proprietary numbered color cataloging system, so you can ensure consistency of your spot colors whoever is printing them. Instead of saying you want something printed in mint green, and hoping the company printing your product knows what you mean, you can say you want it printed in (for example) Pantone 351 C. Unlike CMYK and RGB, spot colors are not a color model. That is, they do not use math to define colors. Spot colors are a color space: a way of cataloging colors. The numbers don’t correspond to a calculation of any kind – they could have used Pantone letters instead, it’s just a way of organizing the colors.

Printing in CMYK Vs Spot Colors

Whether CMYK or spot colors like Pantone are the best option for a printing job depends mainly on the colors you’re trying to produce.

If you’re printing a photo or something with lots of colors, you’ll use CMYK rather than mix all those individual spot colors. But if you’re printing in just a few colors — for example in screen or pad printing — spot colors are more accurate and more vibrant than CMYK.

Spot color inks are typically more expensive than CMYK inks, and because each ink is different, you have to thoroughly clean the plates in between uses. But if you only print in one or two colors and you’re printing a large run, buying that exact ink rather than relying on a CMYK mix can end up being more cost effective in the long run.

One time when spot inks really shine is in logos and other branded goods. Brands are extremely fussy about getting their logos exactly the right color, and that’s where a color matching system like Pantone is essential for making sure the ink is spot on (pun intended.)

The Bottom Line

We tend to take color for granted, but as you can see, there’s a lot going on! CMYK has become the standard color model for the printing industry, since it allows printers to make a wide range of colors using only four inks and some strategic dots. Although it doesn’t always perfectly translate to or from RGB or spot colors, it’s still excellent for printing.