Every now and again, I have to remind myself of the difference between additive and subtractive colour, and which colour mode—RGB or CMYK—uses it. The way I remind myself is with a simple rule called "which way to white.” It works like this: Take a piece of paper that has been printed like an advertisement, presumably CMYK ink printed on a white sheet. How do you get the white colour? Take the ink away. CMYK = subtractive. RGB is therefore additive by deduction. So, CMYK = subtractive, RGB = additive, and "which way to white" helps you remember that.
