"Satellites acquire images in black and white, so how is it possible to create the beautiful color images that we see on television, in magazines, and on the internet? Computers provide us with the answer.
Images created using different bands (or wavelengths) have different contrast (light and dark areas). Computers make it possible to assign 'false color' to these black and white images. The three primary colors of light are red, green, and blue. Computer screens can display an image in three different bands at a time, by using a different primary color for each band. When we combine these three images we get a 'false color image.'
Find tutorials and links to free compositing programs here."
Via Seth Dixon