Adding brightness to an image is very similar to adding light to an image. Similarly, removing brightness from an image is very similar to removing light from an image. The changes in shades seen in the color wheel as light is added or removed are reflected in the way the brightness control works. For example, I'll use the color red:

As light is removed from the color red, it turns into burgundy, and eventually when all light is removed, it turns into black. The effect can be seen on the color wheel by looking at the color red as it moves closer to the black center point of the color wheel. The red becomes a deep red, a burgundy shade of red, and then black. On the other hand, as brightness is added to the color red, it eventually turns into pink, and if enough brightness is added, the pink disappears and becomes white. The changes are reflected in the color wheel as red moves toward the white at the outside of the wheel.

Understanding Primary Colors

As I mentioned earlier, the primary colors in nature are red, yellow, and blue. However, the primary colors on your computer are red, green, and blue. The reason for this is that there are two fundamental type of light, and as we have seen in the color wheel, color is always affected by the presence or absence of light.

The type of light that we think of as natural light is actually defined as "reflective light". Think of it like this: If you're in a totally dark room where there is absolutely no sunlight or other outside light coming in, and absolutely no source of light inside the room, everything is totally black. Sure, the carpeting may be brown and the walls may be beige and the ceiling may be white, but as far as you can tell, everything is black.

Objects in real life settings, such as the carpeting, walls, and ceiling, cannot begin to take on color characteristics in the absence of light. But, as soon as even a touch of light is added, colors begin to appear. This is due to the fact that what we're looking at when we see real life colors is actually reflected light. Light comes from a light source such as sunlight or a light bulb, reflects off of everything it hits, and colors suddenly become visible.

The light that you see from your computer monitor is a totally different type of light. Instead of being reflected light, your monitor is actually a light source. As such, totally different rules of apply. In the case of a light source (such as your monitor) the primary colors are red, green, and blue. In the case of reflected light (such as the color of the wall in the room you're in) the primary colors are red, yellow, and blue.

This difference in source lighting and reflective lighting is why monitors and printers use different color schemes. Since the inks in your printer rely on reflected light to be seen, printers use a color scheme of red, yellow, and blue. Actually, print terminology uses the term "cyan, magenta, yellow, and black", but red, yellow, and blue are still the primary colors. In a printer scenario, cyan is the blue ink, magenta is the red ink, and yellow is obviously the yellow ink. The only way to create black using these colors is to combine all three of them together, which creates a muddy shade of black and wastes ink, so black ink is added to complete the printer color environment.