In the picture above, the left side represents additive color mixing. Directing
the beams of light on to one surface and mixing one or more different
colored lights is a process called additive (light) mixing. The product from
additive mixing is always lighter than any of the individual components because
each light adds energy to the mixture. Any single light color, as long as
it is not a primary color source, can be matched by additive mixing of the
three primary colors of red, green and blue.
In the above picture, the right side represents subtractive color mixing.
Most of us as children learned what subtractive mixing of colors were with
our first box of crayons. Subtractive (substance) mixing occurs when substance
are mixed together and results in the absorbtion of light or energy loss.
This happens when you mix two or more dyes, pigments, or even colored filters.
Since the product of subtractive mixing absorbs light, the product is always
darker than is components. Mixing all three colors together produces
black.