Color terms

Lee Poulsen
Mon, 20 Sep 2004 17:03:29 PDT
On Sep 19, 2004, at 7:43 PM, Jim McKenney wrote:
> I'm surprised that no has yet mentioned the two basic theories of  
> color:
> the subtractive theory and the additive theory. The subtractive theory  
> is
> the one which pertains to the observed behavior of pigments - it's the
> theory painters (and gardeners such as Gertrude Jekyll) used to  
> describe
> color.
> The other theory, the additive theory, gets its name from the  
> observation
> of what happens when light passes through a prism: seemingly colorless
> light breaks up into a spectrum of color. Reverse the process (i.e.  
> combine
> the colors correctly) and you get colorless light again: added rather  
> than
> lessened  brightness, intensity.

I find all this color talk fascinating, and while yes there is a whole  
large area of science and engineering having to do with color  
management, color calibration, and how to make sure that the same color  
is the same color every time it's reproduced on every machine it's  
reproduced on, there is a basic theory underlying it all. (I hope Mary  
Sue doesn't mind  all this talk about it. But after all, we're all  
interested in the various colorful flowers that our bulbs produce and  
with the wiki and digital cameras, this group also spends considerable  
time sharing photos of these flowers and viewing them, and talking  
about them.)

The two theories of color (additive and subtractive) that Jim M.  
mentioned are both part of the same overall theory, really. You can  
think of the additive theory as being (computer) monitor color theory  
and subtractive theory as being color printer color theory. Additive  
color theory pertains to colors produced directly by colored light  
sources, such as the little phosphor dots or LCD dots (if you use a  
flat screen or laptop) covering your monitor screen. Subtractive theory  
pertains to colors produced by reflecting certain frequencies of light  
from pigments (such as color toner or ink dots on a sheet of paper from  
your color printer) when light containing those frequencies shines on  
them. (White light is a mixture of all visible frequencies and is  
therefore the best light to view all colors under. You don't need light  
to view colors produced directly using light.)

What I've sort of drawn below is the CIE color space that is based on  
human color perception. The outer horseshoe-shaped figure represents  
the location in human color perception space that each frequency in the  
visible spectrum falls. In this representation it marks the limits of  
the most intense hue a human would perceive if light of that frequency  
shined on the retina of his/her eye. All other colors (that humans can  
perceive) lie within the interior of this shape. What's nice about this  
shape is that if you draw a straight line between two locations on the  
outer shape, then pick a third location somewhere along that line  
between the two outer points, that will be the color that a human will  
perceive if you mix proportions of each endpoint color in the same  
relative amounts as the ratio of the two distances along that line from  
each endpoint to your designated third point. If you mix fairly equal  
amounts of color from three points, the topmost point and each of the  
two lower corners (which corresponds to mixing green, blue, and red  
light together), you will get white (or shades of grey fading down to  
black). [This diagram doesn't show what happens as you lighten or  
darken a color; you need a 3-dimensional plot to show that. It only  
shows the case where everything is at its most intense.]

What I've drawn in the middle are two triangles that are supposed to  
represent in case 1, with the point going upwards, the locations of the  
colors of the three different phosphor dots or LCD dots (marked R, G,  
and B for red, green, and blue) that cover the front of a monitor, and  
in case 2, with the point going downwards, the locations of the colors  
of the three types of color toner or inkjet ink (marked with C, M, and  
Y for cyan, magenta, and yellow). [Most color printers also have a  
black ink, designated K, since mixing equal amounts of C, M, and Y  
tends to produce a dark grey or dark muddy brown and the black ink or  
toner allows you to print really nice blacks.] The points of the two  
triangles can be considered the primary colors for that medium. Notice  
that you can produce the three primary colors of the opposite medium by  
mixing together pairs of the primaries of each medium, but never as  
intense a version as the pure primary of the opposite medium.

Notice that in either case, no part of the triangles is as intense as  
pure light shining on your eye (the outermost curve). What the  
triangles represent is the boundary of all the colors that can possibly  
be produced with just those three colors (RGB for monitors, CMYK for  
printers). Also notice that monitors can produce a larger number of  
colors and more intense hues than inks or toners can. These two  
triangles also show why you can't ever faithfully print all of the  
colors that show up on your computer screen because parts of the  
monitor's triangle lie outside the printer ink's triangle. (There are  
also a smaller number of colors that can be printed that don't show up  
as well on a monitor.) (Unless you can find phosphor colors or LCDs  
that form a monitor triangle that completely encloses the printer  
triangle which often tends to be smaller.)

BTW, I've also written in the two other color terms that Rodger  
mentioned that fall on the outermost shape, orange and purple. Pink is  
basically a less intense magenta, and brown is a darkened, less intense  
red/orange. Black, grey, and white lie at the dot in the middle.

BTW2, the straight line between the blue endpoint and the red endpoint  
is called the purple line or the nonspectral color line. These are  
colors that cannot be induced in the human mind with a single frequency  
of light. They all require at least two different frequencies  
simultaneously shining at the same point. Thus, you will never see  
these colors in a spectrum produced by a prism or in a rainbow since  
both of those separate out all the individual frequencies. It goes from  
kind of the rose-reds through the pinks and magentas to the purples,  
lavenders, violets to the blue-violet. The corner of the blue side of  
the curve is a deep ultramarine blue.

BTW3, it's kind of funny that so many languages merge the greens with  
the blues given that it covers so much territory in color space. Maybe  
that's why there is no natural word, a la what Rodger told us, for  
cyan. However, these days in America I can use the word teal, and most  
younger people, usually, know what I'm talking about. And they don't  
confuse it with "true" blue. As for magenta, many people just think of  
it as "hot" pink (i.e., a really intense pink). Maybe cyan/teal is less  
understood because there are so few things that occur naturally in  
Western society that are in that color range. (Such as tropical ocean  
shores with white sandy bottoms, or Lachenalia viridiflora or Ixia  
viridiflora or Puya alpestris or P. berteroniana or Strongylodon  

--Lee Poulsen
Pasadena area, California, USDA Zone 9-10

                                 CIE Color Space

                                         -----  ---
                                      /--          --
                                    //               \
                                  //                  \
                                //                    \
                              //                       \
                            //                          \
                           /                            \
                          /                              \
                         /                               \
                        /                                 \ yellow
                  cyan /                   G               \
                      /          additive  \               \
                     /            colors  / \               \
                    /         (monitors) /   \              \
                    /                   /     \              |
                   /                   /      \              |
                  /                   /   g    \        Y    |
                  /      C          /     ------\------/      |
                 /        \--------/------       \    /       |
                 /         \      /             y\   /        |
                /subtractive\   / c               \ /          | (orange)
                /  colors    \ /                   X           |
               / (printers)   X                   / \           |
               /             / \        white     /  \          |
              /            /    \         *      /   \          |
              /           /      \        K     /     \          |
             /           /        \  (or black)/       \         |
             /          /          \          /         \        |
            /          /          b\          /r        \         |
            |        /              \        /           \        |
            |       /                \      /             \       |
      BLUE |       /                  \  m /               \      |
           |      /----------------------------------------\      |
           |     B                      \/                  R     |
           |                             M                        |
          |                                                       |
          |                                                       | RED
               (---purples---)       magenta

        In  1931, the  CIE (Commission  International de  l'Eclairage)
       developed an international standard  of color by measuring  the
       human perception of wavelengths of visible color.

        The CIE "triangle"  is a horseshoe  shaped schematic of  color
       wavelengths ranging  from  around 400  millimicrons  (blue)  to
       around 750  millimicrons (red).  This  is laid  out on  an  x:y
       coordinate  system  based on measured  human color  perception.
        Pure  colors  are  arranged  on   the  outside  of
       the triangle,  with  white  light in  the  middle.  Using  this
       coordinate system, virtually all visible colors can be mixed.

        Green is  placed at  the top of  the CIE  triangle (being  the
       middle wavelength, around  520 millimicrons), moving  clockwise
       to red on the right, variations of magenta to violet along  the
       bottom line, blue on the bottom left, and cyan on the mid-left.
       The placement of the colors is based on color temperature,  and

        (Interestingly,  this  is  the  color  space  upon  which  all
       computer based color systems  operate. RGB color is  calculated
       from CIE Lab color, and when RGB color is converted to CMYK for
       printing, it first  must be  translated through  the CIE  color

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