Color terms
Lee Poulsen (Mon, 20 Sep 2004 17:03:29 PDT)

On Sep 19, 2004, at 7:43 PM, Jim McKenney wrote:

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
macrobotrys.)

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

========================================================================
======
CIE Color Space

GREEN
--
----- ---
/-- --
// \
// \
// \
// \
// \
/ \
/ \
/ \
/ \ 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
wavelengths.

(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
space.)