There are two basic ways to create colors in physics. One way is to start with nothing (black) and add colors to create a composite color, which is the additive method. The other way is to start with all the colors (white) and filter out what you don't want, which is the subtractive method.

Subtractive color is what happens when you mix paint, print a picture, or highlight a word on a page. This is the type of color mixing we're all most familiar with. "Yellow plus blue equals green" is a subtractive color mixture (actually it's "yellow plus cyan equals green", but we'll get to that in a minute).

Additive color occurs with televisions, computer monitors and stage lights. This way of mixing colors is usually less well understood. It's easiest to explain the two methods with a diagram. The following diagram shows the primary colors of both methods and shows how they mix together.

Subtractive	Additive
Key: R=red, Y=yellow, G=green, C=cyan, B=blue, M=magenta, W=white, K=black.

There's a lot to talk about with this diagram. Let's just start with the left side, subtractive. The first thing to note is that our elementary school art teachers lied to us when they said the primary colors are red, yellow, and blue. Actually, they're cyan, magenta, and yellow. Mixing red, yellow, and blue with one another tends to make very muddy colors. For instance, mixing yellow and blue tends to make a gray mess, not a vibrant green. Something else you may notice is that cyan, magenta, and yellow are the colors of the ink in your color printer. Using these three inks (plus black to help with very dark colors) a printer can create nearly any hue.

Subtractive color, as its name implies, subtracts color when mixed. In the diagram above you can see that each mixed color (R, G, B, and K) is darker than any of the primary colors (C, M, and Y). Every time a color is added less light is reflected, making the mixed color appear darker. This is because each primary color absorbs a certain part of the color spectrum. When you mix all three primaries together they wind up absorbing the entire spectrum, leaving no light left behind to hit our retinas (which we perceive as black).

Now let's talk about additive color. Additive color is an exact inverse of subtractive color. The secondary mixed colors we mentioned above (red, green, and blue) are now the primaries. When you mix red, green and blue light you wind up with cyan, magenta, and yellow (and white, if you mix all three at once). Every time another color is mixed in the resulting color is brighter.

If additive color isn't quite clicking, here's a good hands-on way to experiment with it. Get three flashlights and some red, green, and blue transparent plastic or film. Deep, rich colored film will do much better than pastel colors. Cover each flashlight lens with a different color of film and experiment with shining combinations of the three lights on the same spot on a white wall.

Finally, you may have noticed in the diagram that the colors in the additive and subtractive diagrams don't quite match up. This is because it's harder to make ink perfectly cyan or perfectly yellow than it is to make a light perfectly red. This is why sometimes when you print a picture on a color printer the greens come out rather muted and bright colors sometimes come out dark. For this reason, computer monitors tend to be able to display more colors than a printer can. The range of colors a device can display is called its color gamut.