BecauseRobots

If you’ve spent any time messing around in Photoshop, the GIMP, Illustrator, Inkscape, or any other art or drawing program, you’ve probably encountered these two. RGB is fairly simple to explain but CMYK seems to be seldom explained at all.

RGB is simple– its the brightness or amount of Red, Green, and Blue that make up a color. Sometimes its on a scale of 0 to 100 (Windows for example), sometimes its on a scale of 0 to 255 (or FF in hex.) 0 to 255 is a truer representation typically, as most computers use 32 bits to display colors (check your Display Settings), where 8 bits are used for each color (that’s 2^8 or 256 (0 through 255) different levels) and the last 8 bits are used for transparency.

RGB is an ‘additive’ scale, as in all three at 0 is black, and all three at full is white. This is as opposed to…

CMYK is a ‘subtractive’ scale. There are four colors (Cyan, Magenta, Yellow, and the Key plate color Black.) For simplicity, we’ll say that each can go from 0 to 100, although 0 to 255 again is a ‘more accurate’ scale.

So why four and not three? Because for printing purposes, you don’t generally want to overlay full cyan, magenta, and yellow ink to try to make black– its much cheaper and more effective to use black ink.

So, for example, after converting a color for RGB to CMY, say you got 20 cyan, 40 magenta, and 60 yellow. Instead of printing that, we ‘take’ out the ‘grey’ and rescale the remaining colors. Cyan is the least used here, so we’ll use 20 black and make cyan 0. Now our scale goes from 20 to 100 instead of 0 to 100, so we need to ‘stretch’ the remaining colors to get their adjusted brightnesses. For magenta, (40 – 20)/(100 – 20) is .25, so the new magenta will be .25 * 100 or 25. For yellow, (60 – 20)/(100 – 20) is .5, so the new yellow is 50.

That’s right, 20C 40M 60Y 0K is the same as 0C 25M 50Y 20K in a true sense. In a practical sense, the printing result will be different because the latter will be using less ink overall and black ink instead of cyan. In print, usually less ink is better and black ink shows up sharper than ‘mixing’ the other three, so this is always a good thing.

Most programs will happily do the conversion for you. Note that these formulas do round a bit, so be mindful and if you’re going to do a lot in print make sure you record what CMYK colors you’re going to use for important things like logos so that when you let programs convert you can make sure they always convert to the same color.

Tags: graphic