Question

Why is it that a computer monitor or a TV screen can produce millions of colors using only the 3 primary additive colors?

Likewise, how can a printer print millions of color using only the colors of magenta, cyan, and yellow?

How are the primary additive and the primary subtractive colors related?

Answer 1

This has little to do with physics. THe colors don't really add, but the brain senses a new color. The (human) eye consists of three types of cones, each sensitive to a different color range. When colored light strikes the eye cones, the sensations sent to the brain gives sensations of a combination of color, which the brain registers as a "single" color. Differences in how the eye cones react to different colors also accounts for some color "blindness", and why some humans see some colors differently. So you question is really a biological one, asking how humans are different from other animals, how human eye cones vary from person to person, and how the brain classifies information (and interprets it).
http://en.wikipedia.org/wiki/Primary_color