In the last post, we looked at colors and some different color schemes that designers use. Prisms also made an appearance, as did school paints. In school, we learned that prisms took white light and turned it into a rainbow, but a rainbow of school paints, mixed together, becomes black.

This observation can help us understand additive and subtractive color, and is why color on a computer screen behaves differently than color on a printed piece.

The place to start when understanding additive color is with the prism. Once the white light passes through the prism, the light separates into the visible spectrum of colors — red through violet. Each color is a different wavelength of light.

We can take just one wavelength, e.g. the wavelength that makes up green, emit it and we'd see the color green. Computers and TVs use emitted light to make color. On a TV, the pixels start out black. This is because no light is being emitted from them. To produce red on a TV screen, red light — and red light only — is emitted. The same applies to green and blue. All other colors are derived from some combination of these colors. White, of course, is all the colors being emitted together. Producing color like this, where different wavelengths are being "added" together, is called additive color. The color we see is the light shining out at us; think of that as you're basking in the computers glow.

Subtractive color is a bit more complicated. Let's start with an example. If you're looking at a red apple, you're still seeing with light. But in this case, the red apple isn't emitting light; rather it's absorbing all wavelengths of light except red, which is bouncing off it. To think of it in another way, it's as if the apple is accepting all comers and absorbing them, except red, which it's throwing back at us.

Printed ads, paintings, most of the items we see around us we see because of subtractive color. When children mix all their paints together they get black because all the light is being absorbed and none of it is bouncing back at us.

So you can imagine getting the inks right on a printed ad is important; if the mix is a little off it will absorb and reflect the wrong set of wavelengths and we'll see (slightly) the wrong color. This is one of several reasons why we see variations in the colors of printed materials — slight differences in the pigment mix.