Why does red mixed with green make yellow?

When you mix paint, it's a subtractive process. You start with white, and you're putting a layer of something on top of it, which allows a certain wavelength of light through but takes away everything else. So whatever you add, it always gets darker, and changes colour in a certain way. When you mix light, you're starting with black, and you're adding something. If you think a the way a TV works, it sends out red and green and blue and makes a coloured picture. But it's adding something all the time. Paint is always taking something away. It's a fundamentally different way of doing something. An interesting aside to that is that when you take a picture with your camera, it records red, green and blue. When you print it out, the printer has to convert it into something called CMYK, which is cyan, magenta and yellow. These are mixed to make the colours that you see. Printers use a different palette because when you add things subtractively, you end up with different colours. Interestingly, the K is black. In theory, if you mix the cyan, magenta and yellow together, you get black, although in practice you don't quite, so they have to add black to make everything work.