Colors. Without them our world would be gray. Different shades of gray (and definately more than 50), but gray. Some humans are able to differentiate between a few thousand, or even ten thousand of colors, and yet we only have receptors in our eyes for blue, green, and red light. Some people are colorblind, and are missing some of those receptors, or even all.
We perceive certain colors depending on how the reflected light of an object interacts with our receptors. Yellow light, that is reflected from a yellow object, interacts with both our red and our green receptors. From that our brain determines that that color must be yellow. When mixing colors, we use this fact to our advantage to fool our eyes. Look at this yellow rectangle, for instance:
If we zoom in ... closer ... even closer ... then you can see that your screen doesn't have yellow pixels. There are only blue, green and red (sub)pixels. And all the colors we see on the screen are mixed using these three base colors.
From school, or dabbling with physical art, you might remember that you had three different base colors: Blue, Red, and Yellow. What is going on here? The answer is quite simple: Look at a piece of paper. Anything colorful on a piece of paper works by the same principle: The colors absorb part of the light, and only reflect the parts it can't do anything with. In fact, the complimentary color is missing. For example: Something that appears yellow absorbs blue light, that is: It eats all the blue light, and let's everything else go. Something that appears cyan absorbs all the red parts of the light, and something that appears magenta gobbles up all the green light.
Creating colors by absorption is called subtractive color mixing. You take out as many pieces of light until you get the color you desire. This is the type of color mixing that is used in print. Every printer therefore uses these colors to create (almost) every color: Cyan, Magenta, Yellow, and BlacK (CMYK). Black is needed, because mixing of all colors in to black is theoretically possible, but the result is for one unsatisfactory, and otherwise needs three times as much ink, or toner, as using a single black.
Any Display or Monitor mixes it's color by adding colors together. They emit light. More and more light is added to a color until you get the color you want - even up to white. These are Red, Green, and Blue (RGB).
Every picture you make, is usually made in the RGB-color space. Any old Camera is using this color space, and the pictures need to be converted into CMYK, if you want to print them. There are formulae for this, and programs do this automatically. But there is a caveat to it.
You might have seen in tests for displays, that this one fills a 99% sRGB, and 82% adobeRGB. The dream display for anyone working with graphics or photography would be a display that fills almost all of the adobeRGB color space. That means it would come the closest to displaying all colors that nature offers.
If you are somewhat like me, then you're fully satisfied with 99% sRGB. Now that we have established that sRGB is smaller than adobeRGB, we can have a look at CMYK. Sadly, CMYK is even smaller than sRGB. This means that some colors can't be reproduced in CMYK, which can be a problem for any picture you want to print: Pictures are usually saved with RGB colors and need to be converted to the smaller color space of CMYK. This can lead to the problem that these pictures (especially logos) may look very different in print than they do on screen. Depending on how they are converted, they may even differ from printer to printer.
Rendering Intents can tell a printer how the colors should be converted into CMYK. There are four Rendering Intents - and a desktop publishing program (like Scribus) lets you choose between them.
I'll talk more about what those do in another post, as I will have to prepare a few images in order to explain them properly.
