Color Theory – Additive and Subtractive Color CMYK/RGB

Understanding how color works is very important when working with any graphic product. In this article I’ll talk about the basics of color creation and make a simple explanation of the two color systems currently in use: RGB and CMYK.

What is color?

Before we talk about color systems we need to know what it is. Perhaps the most basic definition would be to say that color is simply light. In fact it is true, color is nothing more than light perceived at different wavelengths of the visible light spectrum. The color we see in the physical world is nothing more than light reflected or bounced off the surface of objects.

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Subtractive and additive synthesis

When we speak of subtractive and additive synthesis we are referring to two different forms of creation and color perception. The digital world and the physical world each have their own way of creating color.

Additive Synthesis

Every color represented on a screen uses the additive synthesis to be created and represented so that we can see it. Therefore every color represented in a digital environment uses this color creation system.

It is called additive synthesis because, explained in a simplified way, colors are created from amounts of “added” light in each pixel. Each pixel is made up of 3 colours Red Blue and Green (RGB red, green, blue). Each of these colors can have an intensity that is measured from 0 to 255, being 0 the absence of light and 255 maximum light. Each of these colors represents a channel. That’s why in Photoshop we have a panel of Channels that shows us the separate image in the 3 color channels.

If the colors of the three channels are at maximum power (255) the result is white, or put another way, our pixel will be white.

Subtractive synthesis

The colors formed through subtractive synthesis are those that are formed in the physical world through the process of subtraction, hence its name. It is the opposite process to additive synthesis. The spectrum of visible light could be defined as white light. It is the light that contains all the colors of the rainbow.

In the physical world the surface of objects absorbs a certain amount of light (part of the visible spectrum) and reflects part of it. This reflected light is what the human eye perceives. For example, objects that we perceive as white are objects that absorb virtually no light and reflect almost all of it.

In other words, they reflect almost the entire spectrum of visible light and as a result we perceive white light and therefore perceive the object as white. For example, A blue object absorbs all the light in the visible spectrum and bounces those wavelengths corresponding to the blue, which is what we see.

It is called subtractive synthesis because objects take away part of the light. It is a behavior typical of pigments. Every surface is composed of matter that has pigment that gives color to things.

To better understand the subtractive synthesis let’s take painting as an example. If on a white paper or canvas we begin to paint a circle of one color and with each brushstroke we add a new color on top of the previous one, the color tends to darken, tends to black. That’s because each pigment absorbs part of the light that reaches the point that if we paint with all the colors, the pigments absorb almost all the light and reflect very little. It is the natural behaviour of pigments, which are the ones that give colour to matter and printing inks.

In the case of additive synthesis the process is the opposite, remember that these colours are created from coloured lights and not from pigments or reflected light. An easy way to prove it is by using three flashlights of Red Blue and Green colors. If you superimpose the beams of light of the three lanterns you will obtain a white light.

It can also be checked in Photoshop using the Fade or Lighten mode. When you add more colours, the resulting colour will tend towards white.

Relationship between CMYK and RGB

It must be made clear that both RGB and CMYK are simply colour models. The combinations of colors that each model forms gives us a certain number of possible colors that we call color space. The RGB color space produces more vivid colors on electronic screens. The CMYK color space represented on an electronic display produces more desaturated effects on colors. Take a look at the following example.

Photograph in CMYK color mode without additional color treatment
Same picture in RGB mode. It presents a greater Saturation and vividness in the colors.

Keep in mind that the RGB photo can never be reproduced with the same level of vividness and color saturation as the original version by multiple factors. The relationship between RGB and CMYK systems is that one is complementary to the other, ie combined two primary colors of the RGB system we get a primary color of the CMYK system. For example if we mix Cyan ink (C) and Yellow (Y) we get Green (G).

Why printing uses the CMYK color system?

CMYK works with inks printed on any material through an offset machine or in digital, and RGB is a simulation of these inks created through artificial light on digital monitors and screens. The reason why the prints are made in CMYK is because it is the model that best suits to represent the colors on the media.

Previously the RYB model was used (Red Yellow Blue) but the CMY model improves color fidelity and above all produces more intense blacks. If the combination of CMY produces black, why we have the K? The K of the CMYK model has been introduced for several reasons. The main reason is because the black obtained from the mixture of Cyan Magenta and Yellow is not a pure black and therefore had to introduce a fourth color that is a pure black.

The second reason is to reduce printing costs and registration errors. If we have to print black text, it is much more expensive to pass the support through 3 plates to obtain a black that is not even a pure black instead of passing it through a single body that produces us directly the black and thus avoid registration errors. The registry errors are those that are generated when the colors do not overlap perfectly. (more information about the CMYK printing system with the offset system)

Simulated representation in Photoshop of how a text printed on 3 plates ofset with the 3 CMY inks (top text) would look like and the result when printing the same text only on the black printing plate.

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I'm a self-taught Photoshop artist and like teaching other how to improve their skills. I love design and graphic communication. I've been creating Photoshop tutorials since 2009 for beginners and advanced users

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