Television screens and computer monitors use color addition to create the many colors you see on the screens. If you look very very closely at a monitor or TV, you will see many sets of tiny red, blue, and green lights. When these lights combine and strike your eye, you interpret the red, blue and green combination to be color.
The "Color" White
White is the combination of all colors of light. In other words, the color white is seen when the red, green and blue channels all send signals to the brain. It follows that red light plus green light plus blue light produces white light. However red plus green without the blue produces yellow. By deduction, yellow light plus blue light most produce white light.
Colors that combine together to form white light are called complimentary colors. . There are many sets of complimentary colors such as blue and yellow, green and magenta, and red and cyan. Notice that these colors are completely opposite: there is no such thing as a blueish yellow, greenish magenta, or reddish cyan.
Complimentary colors are the colors seen in negative afterimages. If you stare at the color green for a long time, your "green" channel is desensitized. When you look at something white, the green channel doesn't respond as strong as it would under normal circumstances, so the white appears like green's compliment: magenta.
The ideas of color addition create a very interesting question about the way we view the world. We perceive red and green together as being yellow, however yellow by itself is also a spectral color. Polychromatic red plus green may appear the same as monochromatic yellow, but the mixture is not actually the same color.
Light combinations that appear the same but are actually different are called metamers. Metamers exist because we only have three color channels to determine color. If we had more channels there could be more color signal combinations and we could see more colors.
A Word on Subtraction:
Many people reading this may be shocked that we called the primary colors red, green and blue. Everyone learns in preschool that the primary colors are red, yellow and blue. This set of primary colors is also true (almost). These are the primary colors of pigments, not of light because of the process of color subtraction. Pigments are the dyes, paints, oils, and inks that can be used to color items. When light hits a pigment, some of it is absorbed, or subtracted. When white light strikes a black surface it is all absorbed so no light reflects and hits the eye. When white light strikes a green surface the red and blue are subtracted, leaving green light to strike the eye.
When different pigments are mixed, they absorb different wavelengths of light. Blue pigment absorbs red and green, while red pigment absorbs blue and green. When the two are mixed, they absorb most of the green, and some blue and red. The blue and red that reflect add to form a magenta or purple color.