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Your color monitor screen, and your color TV screen, are made up of millions of tiny colored dots (or stripes). They are colored red, blue, and green. Why is that?
Before I explain it, let's see how colors are mixed on the screen:
| black | none of the 3 colors |
| red | red |
| violet | red+blue |
| blue | blue |
| blue-green | blue+green |
| green | green |
| yellow | red+green |
| white | red+blue+green |
Now, if you've mixed paints, you know that red+green do not make yellow. They make a kind of dark brown. Mixing paints is a subtractive operation. Red pigment absorbs most of the light that hits it, and reflects mostly the red light. It subtracts out most of the light, and leaves red. It does reflect some of the other colors, too. Similarly, green pigment absorbs most of the light, and reflects mostly the green light. Well, mix red and green, and most of the light is being absorbed.
Mixing light is an additive operation. And this is not really a property of light, but instead is a handy property of your eye. The retina of your eye (an outgrowth of your brain actually) is made up of light-sensitive cells call rods and cones. The rods are not involved in color vision, they just detect the amount of light. The cones detect color, and do not work well in low light. Some cones have a small amount of red pigment in them, they detect mainly red light. Some have green pigment, and detect mainly green light. And a few have blue pigment, and detect mainly blue light. If you are missing any or all of these cells, you have impaired color-vision, and you are color blind. For example, some people have no cones which detect blue light.
When yellow light enters your eye, it falls on blue cones, and is mostly undetected. It falls on red and green cells, and is detected quite a bit more. Your brain interprets this combination of signals as yellow. The original light was pure yellow, but the signals arrive at the brain as red and green. Your brain is smart enough to know that this means yellow.
A TV camera acts much as a human eye. With the use of red, green, and blue pigments, it too converts the yellow light into signals meaning red and green. The TV station sends these red and green signals through the air using radio signals, or through your cable. And an electron beam in your TV screen shines on a red dot, and a green dot. This red and green light hits the retina of your eye, and is detected by red and green cone cells. And your brain makes the conversion to yellow, even though not much yellow light entered your eye.
We are told that a matador's cape does not have to be red, because bulls are color blind. You may ask yourself, "How do we know that?" No, they didn't hold one of those color blindness test booklets up in front of a bull to see if he would charge. Once upon a time, someone cut open a bull's eye, and, under a microscope, saw that there were just rods, no cones. Aren't you sorry you asked?
Addendum:
It may seem hard to believe that red and green can make
yellow. On the left is proof. The tiny squares on the left are red and green,
and seen from a distance (or if you squint), the entire picture looks yellow.
But no yellow light whatsoever is entering your eye. The same kind of mixing of
many of the colors is done in comic books, except that they use yellow ink to
get a brighter yellow.