Colour is interesting area of print and can be really hard to get your head around, and after seeing this video you’ll understand both why it’s hard and how it works. A quick note: the colour mixing they’re doing here is with light and that’s the additive colour process. In print using inks, it’s done with a subtractive colour process using cyan, magenta (which apparently doesn’t exist!), yellow and black. For a bit more information on that, take a look here on Wikipedia.
Take a look and enjoy.