Color requires our specifying 
three variables; to any color 
there corresponds a triplet of 
numbers, and we can think of 
any color as occupying a point 
in three-dimensional space. 

Top: Land's original formulation 
of the color-constancy problem 
seems to call for three kinds of 
cells, which compare the 
activation of a given set of 
cones (red, green, or blue) in 
one region of retina with the 
average activation of the same 
set in the surround. The result 
is three numbers, which 
specify the color at the region. 
Thus yellow, brown, dark gray, 
and olive green each has a 
corresponding triplet of 
numbers. We can therefore plot 
colors in a color space specified 
by three axes, for red, green, 
and blue. Bottom: A 
mathematically equivalent 
system also gives three 
numbers, and is probably closer 
to the way the brain specifies 
color. At any point on the 
retina, we can speak of red-
greenness, the reading an 
instrument would give if it 
were to record the relative 
stimulation of red and green 
cones (zero for yellow or white). 
This value is determined for a 
particular region, and an 
average value is determined for 
the surround; then the ratio is 
taken. The process is repeated 
for yellow-blueness and black-
whiteness. These three figures 
together are enough to specify 
any color.

    We can plot points in such a 
space in more than one way. 
The coordinate system can be 
Cartesian, with the three axes 
orthogonal and oriented in any 
direction or we can use polar or 
cylindrical coordinates. The 
Hering theory (and apparently 
the retina and brain) simply 
employ a different set of axes to 
plot the same space. This is 
doubtless an oversimplification 
because the blob cells making 
up the three classes are not like 
peas in pods but vary among 
themselves in the relative 
strengths of surrounds and 
centers, in their perfections in 
the balance between opponent 
colors, and in other 
characteristics, some still not 
understood. At the moment, we 
can only say that the 
physiology has a striking 
affinity with the 
psychophysics.

    You may ask why the brain 
should go to the trouble to plot 
color with these seemingly 
weird axes rather than with the 
more straightforward r, g, and b 
axes, the way the receptor layer 
of the retina does. Presumably, 
color vision was added in 
evolution to the colorless 
vision characteristic of lower 
mammals. For such animals, 
color space was one-
dimensional, with all cone 
types (if the animal had more 
than one) pooled. When color 
vision evolved, two more axes 
were added to the one already 
present. It would make more 
sense to do that than to throw 
out the pooled system already 
present for black-white and 
then have to erect three new 
ones. When we adapt to the dark 
and are using only our rods, our 
vision becomes colorless and is 
again plotted along one axis, to 
which the rods evidently 
contribute. That would not be 
easy to do with r, g, and b axes.