Color is a part of the human vision system.
When the image of the apple's red skin is focused at the back of the eye, it affects red sensitive cells to a greater extent. All other colors affect these receptors to varying degrees so that we are able to discern a great range of different color mixtures. A prism can be used to show that white light is really a bunch of mixed colors. Although the spectrum shows that white light contains a continuous range of colors, colors from just three areas of the spectrum could be mixed to form any other color. These three colors of light are from the red, green, and blue parts of the spectrum.