I got one one of these extremely useful babies from Galaxy Gauge to help me chose accurate colours for the backgrounds of the latest illo project:
It tells you exactly how much C, M, Y or K to put together in Photoshop (or similar) to make specific colours. See that "Pine" green up there? If you open the Colour Picker tool in Photoshop and put in 86% Cyan, 20% Magenta, 86% Yellow and 10% Black in the boxes provided on the right hand side, you get that particular shade of green. Cool, huh? And you know that's how it will print, too. Super awesome and useful, love it. Can't believe I didn't get one before now.