What's the deal with RGB, CMYK, pixels and DPI?

Get the low-down on colour and images

RGB and CMYK

So what are RGB and CMYK? Well they're all about colour and how you're going to use it. If I ask you "what are the primary colours?" and you answer me "red, yellow and blue", then you need to keep reading.

Franciscus Aguilonius created that colour model in 1567, and it really annoys me that schools still teach such outdated information. The idea of "primary colours" is one where you find the essential colours that can make absolutely every other colour. Did you ever notice how, when mixing paints in school, the results were rarely desirable? You could create orange or green well enough but when you tried to make to purple, the result was always really dark?

Since the 16th century, lots has happened in the world of science. The work of Issac Newton and many others has given us the colour model RGB. By mixing different coloured lights, it was discovered that a much more satisfying set of primary colours is red, green and blue. Just like when you split white light up to get a rainbow - in reverse, you could add all of those colours to make white again, or get the same result by blending only red, green and blue light. So because all the primaries add up to white, RGB is called an "additive" model. And if you look really closely at your tv or computer screen, you'll see the magic of RGB at work.

When you mix red and blue, you get magenta. Blue and green make cyan. And green and red make yellow. And interestingly, when you take cyan, magenta and yellow, you make another ideal colour model. Especially so for printing. When you work with light, you build up from dark, but with printing and painting, you're usually building from a base of white (paper) and going darker. So cyan, magenta and yellow make up a set of "subtractive" primaries. And all of them together make black. If you've ever seen an ordinary colour printer cartridge, you may have noticed it uses CMYK. that's Cyan, Magenta, Yellow, and blacK (black is K, because B was already being used for Blue). Black was added to the printing model because it's so common, and so it is always printed sharp and clear (instead of relying on all of the colours aligning and mixing perfectly).

Pixels, DPI and Resolution

What's a pixel? When you looked up close at that computer screen, every group of r, g and b makes one pixel. When you hear of PPI (pixels per inch) or DPI (dots per inch), it's practically the same thing. Printers use dots, and screens use pixels. If you look at each up close though, you will see that screens can get away with a lot fewer pixels/dots than a print can. Resolution means how many dots or pixels there are per inch.

There are many exceptions, but generally images for screens use 72 pixels per inch, and images for print will need to be more than three times larger (often 300 dots per inch) to achieve the same level of clarity.

As a general rule, if a picture looks great on your screen, it'll need to still look great at three times the size (zoomed in 300%) if it's going to work well when printed at about the same size.