A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

If blue, red, and yellow are primary colors, why do color TVs use blue, red, and green?

July 26, 1991

Dear Cecil:

As kids we were taught in art class that the primary colors were red, blue, and yellow. By mixing these primary colors, we were told, we could come up with any color of the rainbow. Experiment seemed to bear this out.

Now that we are older and in the age of video, we have been told that color TV monitors use red, blue, and green as primary colors. The obvious questions are, how do you mix red, green, and blue to get yellow? And why can't I replicate the feat with my daughter's crayons?

Cecil replies:

When I was a little sprite this bugged me, too, so I asked Mr. Grayson the science teacher about it. His response was to bring in a power drill with a red, green, and blue color wheel attached to the tip. When he pulled the trigger the colors on the spinning wheel merged into a sort of light gray. Nobody in class had the faintest idea what this was supposed to prove. However, it did have the effect of making Mr. Grayson, a bespectacled, slightly buck-toothed fellow, look like Flash Gordon on acid. So we considered it an afternoon well spent.

It was only later that the significance of the color wheel demonstration dawned on me. It turns out there are two ways of creating colors — the additive method and the subtractive method.

When using the additive method, the primary colors are red, blue, and green. The more additive primaries you add, the lighter the resultant color. Mix all three and you get white. The subtractive primaries are red, blue, and yellow — to be exact, magenta, cyan (light blue), and yellow. These are the colors that, together with black, are used in color printing. The more subtractive primaries you mix, the darker the color. Mix all three and you get black (OK, brown, but with kindergarten paints you can't expect miracles). As a general proposition, additive primaries involve adding more LIGHT (as in a color TV), while subtractive primaries involve mixing more PIGMENT (as in paints and crayons).

Additive colors are easy to demonstrate on a color computer monitor equipped with a color-control program. Just so happens I have one right here. How do we make yellow? By adding full-strength red and full-strength green. Adding two-thirds strength blue gives us a lighter (not darker) yellow. Full-strength blue, red, and green produce bright white. This is a counterintuitive result if you learned your color-mixing skills in kindergarten. But we know that white light can be broken into all the colors of the rainbow. So we shouldn't be surprised to learn the process also works in reverse — i.e, the colors of the rainbow can be combined to make white. Besides, it only stands to reason that the more light you shed on something, the brighter (that is, closer to white) it gets.

Proceeding with our experiments we fill out the additive color- mixing chart as follows:

Green + red = yellow

Green + blue = cyan (light blue)

Red + blue = magenta

Red + blue + green = white

Subtractive colors, as the name suggests, work by subtracting certain colors from white light and reflecting the rest, like so:

Yellow pigment absorbs blue — reflects red, green

Cyan absorbs red — reflects green, blue

Magenta absorbs green — reflects red, blue

Blue absorbs red, green — reflects blue

Red absorbs blue, green — reflects red

Green absorbs blue, red — reflects green

If white light strikes yellow paint, the paint absorbs blue and reflects red and green. Then the additive principle takes over — red and green combine to make yellow.

Now mix cyan (light blue) and yellow paint. The cyan pigment absorbs red light; the yellow pigment absorbs blue light. What's left is green, the color you see.

How do you know whether it's additives or subtractives you're dealing with? Ain't easy. Spotlights, TV electron guns, and spinning color discs are additive; pigments, filters, and stationary color discs are subtractive. Confused? Who isn't these days? But perhaps at least you understand the apparent paradox of a TV making light colors from dark ones.

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