RGB Color Cube
If you look very closely at your computer monitor with a magnifying glass you'll see that the surface is composed of a multitude of tiny red, green and blue dots. The way in which these dots interact with each other is how your monitor is able to produce the millions of colors needed to display an image. This interaction can be visualized using a cube, where each axis of the cube represents one of the primary colors. By lengthening one or more of the axes you increase the strength of that component.
In the example above, try moving one of the sliders—labeled "Red", "Green" and "Blue"—to see how it affects the color of the round dot.