No matter how your images got into the computer, whether from a scanner, a digital camera, or copied from a stock art CD-ROM, the version of the image stored in the computer can only approximate the colors of the original scene. A computer, at its core, is only capable of dealing with numbers, so it somehow has to come up with numerical equivalents of the colors perceived by our eyes.
Computers use number systems, called color models to display and reproduce color. One of the most common is the RGB color model. In this model, the color of each pixel is described as combinations of different amounts of the colors red, green, and blue. These colors were chosen because the cells in our eyes that respond to color (called cones) come in three types; some are sensitive to red, some to green, and some to blue. Therefore, the RGB model tries to characterize colors in a way that's similar to the way the human eye perceives them.
It's important to remember that color models, at best, can only approximate the colors in your image. No color model is as sensitive as the human eye.