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 which modal. 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. Therefore, CMYK = subtractive.
RGB is therefore additive by deduction.
The result: CMYK = subtractive, RGB = additive, and “which way to white” helps you remember that.