What is the difference between additive and subtractive colour mixing?

Screens mix coloured light additively (RGB) — combining red, green and blue light at full strength produces white. Paint and print mix pigment subtractively — combining pigments removes more of the light spectrum, moving toward black, and print approximates this with the CMYK model. This is the core reason a colour that looks right on screen doesn't automatically translate to the same result in paint.