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The Straight Dope

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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. A bit of experimenting 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? --Kevin May, Plano, Texas

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. Red, blue, and green are the additive primaries. The subtractive ones are red, blue, and yellow--to be exact, magenta, cyan (light blue), and yellow. Mix the additive primaries together and you get white.

Mix the subtractive primaries together and you get black (OK, brown, but with kindergarten paints you can't expect miracles). Color TVs make use of the additive principle, while the pigments in paints and crayons are subtractive.

The additive primaries are easy to explain. Everybody knows white sunlight contains all the colors of the rainbow. To put it another way, when you mix all the colors (or at least a selected spectrum of colors), the eye perceives the result as white. So it stands to reason the more colors you add together, the whiter (i.e., lighter) a color you get.

Cecil can demonstrate this on his color computer monitor. Full-strength red and full-strength green produces . . . bright yellow. This may seem strange, but it gets stranger. Adding two-thirds strength blue gives us a paler (not darker) yellow. Full-strength blue, red, and green produce bright white--a counterintuitive result, if you learned your color-mixing skills in kindergarten, but that's science for you. Proceeding with our experiments we conclude as follows:

Green + red = yellow

Green + blue = cyan (light blue)

Red + blue = magenta

Red + blue + green = white

Subtractive colors are a little more complicated. Paint and crayon pigments work by absorbing certain colors and reflecting the rest, like so:

Pigment Absorbs Reflects

Yellow Blue Red, green

Cyan Red Green, blue

Magenta Green Red, blue

Blue Red, green Blue

Red Blue, green Red

Green Blue, red 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 let's 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? 'Tain't easy. Spotlights, TV electron guns, and spinning color discs are additive; pigments, filters, and stationary color wheels 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.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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