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Straight Dope Classic from Cecil's storehouse of human knowledge26-Jul-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? --Kevin May, Plano, Texas
Dear Kevin:
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.
--CECIL ADAMS
The Straight Dope / Questions or
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