inside point represents some combination of hue and saturation. The
farther from "C" the greater the saturation. The z-axis (which isn't
shown) is intensity, the necessary third dimension.
It's a "nomograph"!
This is the choice of arrangement
the color theoretician. Here's why: To find what color is produced
by the additive mixture of two colors, simply locate each of the two colors
on the diagram, then draw a line between those two points. The new,
mixed-light color is on that line at a position proportionally located
according to the relative intensities of the two source lights. (The
proportion is inverse, of course. The stronger the light, the closer
the mixture is to it.)
The red lines are colors
that a protanope cannot distinguish between. (This remarkably clear way
to "see" the two dimensionality of protanopia comes from The Feynman
Lectures on Physics I, p 35-8.) That line which goes through
the centroid is—for a "normal"—brilliant, 100%-saturation blue-green at
the left, becomes less and less saturated as we move toward C, and then
shifts into the red and becomes less and less detectable as we approach
the corner of the diagram. For the protanope these are all variations
of white ("shades of gray" if on the printed page). For the protanope,
the two-dimensional chromaticity diagram has all the dimensions of color
collapsed into its two dimensions. Something resembling "hue" is
the angle of those red lines. Or perhaps its closer to "saturation."
Protanopic color does not have three distinct parameters of color for which
the Munsell choice of parameters is meaningful.
A color "normal" person has
difficulty understanding what colors a protanope sees, colors which the
protanope then solves the arrangement puzzle with only two dimensions—and
it is correct for him while not correct for the "normal" person.
Nevertheless, a protanope has much more difficulty understanding what "normal"
color looks like.
A color "normal" person is
no more able to comprehend what a bird sees. Four-component color
must have a fourth Munsell-like parameter in addition to hue, saturation
and intensity. That fourth "dimension" must be as unlike all
of the familiar three as those three are different from each other.
Since what experiencing it would be like would depend on how evolution
developed our brain circuitry to interpret it, we probably cannot imagine
the experience: it simply doesn't exist.
Nevertheless, it represents
real-world phenomena at the edge of human comprehension. To explore
beyond the edge, we need our most powerful modern tool kit. The one
that so often seems to be just a little out of reach:
If you understand a couple
of elementary points from calculus, understanding how "white" arises in
our vision is easy. If you are not familiar with calculus, here is
a relatively easy way to begin to understand a couple of its elementary