If one take two differently colored beams of light and projects them on to a screen, the mixing of these lights occurs according to the principle of additive color mixing. If one mixes two differently colored paints they mix according to the principle of subtractive color mixing.
Select color mixing to see a demonstration of additive and subtractive color mixing.
Select on-line dynamic additive color mixing
Select explanation to see why blue plus yellow paint yields green.
Select trichromatic color mixing to see what happens when red, green and blue lights are mixed.