Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 92

different colors can be identified accurately without training. If you need more than
8-10 distinct colors you may need to use texture or text to provide features to
assist the user in discriminating between them.
People's sensitivity to color is not uniform across their field of view. The eye is
not sensitive to color at the periphery of vision. Accurate discrimination of color is
only possible to around 60 from the straight ahead position (with the head and the
eyes stationary) and the limit of color awareness (as opposed to discrimination) is
approximately 90 from the straight ahead position. The eye is best suited to the
perception of yellow-green light, and color is only well perceived in foveal
(central) vision. It is least sensitive to red, green, and yellow light at the periphery
of color vision where it is most sensitive to blue light. This variation in sensitivity
arises from the way that the rods and cones are distributed in the fovea.
Perceptions of a particular color are affected by prolonged exposure to other
colors—this is because different cones are responsive to different dimensions of
color (e.g., red-green or yellow-blue). Looking at red light, for example, causes
the red cones to become adapted, so the red light reduces in salience. This is often
seen in color after effects, or afterimages. There are also several visual illusions
you may have seen where you first stare at a picture or a display until the colors
apparently disappear, and then when you look at another picture or a blank piece of
paper the complementary colors appear.
Color constancy refers to the situation in which we attempt to perceive colors as
being the same even when they are different. Our clothes do not change color when
we go indoors, for example. The wavelengths hitting the retina may have changed,
however, and the reflected light will be different, although we will still perceive
the colors to be the same. Vegetables and meat in supermarkets are one of the most
compelling examples of constancy not working—stores tend to be lit so that
objects

give off particular wavelengths—when you get home with different

lighting the colors are substantially duller.

4.4.3 Color Blindness

It is important to be aware of color blindness (or, more strictly color vision
deficiency) because around 7% of Western men and 0.5% of Western women are
red-green color deficient. In other words they are bichromats, because they can
only distinguish two primary colors (typically they cannot tell red from green).
Other cues, such as brightness, can be used to help distinguish red from green.
Most of us can discriminate between all three primary colors (red, green, and
blue), and are known as trichromats because we have three types of cones for
seeing color. Quadchromats appear to have better color discrimination under
different lighting conditions, but see the same colors. Many animals are mono-
chromats: they cannot distinguish any colors because they have only one set of
cones (or just rods). They hence perceive the world in monochrome.