Did you know that colours on the computer are different to colours in real life? True facts. The difference is due to dealing with stuff that absorbs/reflects white light vs the colour of the light itself.
Subtractive colour applies to stuff like paint, crayons and other substances that absorb certain frequencies of light – thereby subtracting those frequencies from the light you see. For instance, red paint looks red because all the other light frequencies have been absorbed by the paint, leaving only red to be reflected.
Additive colour refers to mixing lights of different colours, aka. adding them together.
Learn more about it at the site I stole the picture from: http://www.worqx.com/color/color_systems.htm
And if you wanna have some fun, go enjoy this (additive) Color Theory flash game.