Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 97

4.4.7 Implications for System Design
It has long been a rule of thumb that you should use color sparingly on display
screens. In other words, you just use color to emphasize the things that are
important. If your system has displays that rely on widespread use of color, and
there is meaning associated with the different colors, you should check to make
sure that the exposure to the dominant color on the screen is not adversely
affecting the perception of some of the other (possibly more important) colors.
When you do use color to encode items on a display, you need to make sure that
the users can perceive the colors that you are using. Figure 4.13 , for example,
shows how color can be used to help users find information. The artificial horizon
display above the throttle levers has blue sky and brown ground. The center
displays have green and yellow and orange text to help separate information. If
you are designing safety critical systems or interfaces used by a wide range of
users, you should consider using redundant information to help people with red-
green color vision deficiency distinguish between these two colors on the screen.
If you are developing a system which must produce color print-outs, and the
colors and the differences between colors are important, you will have to take into
account the fact that the colors on the screen and on the print-out will appear
somewhat different. You will therefore need to find a way to make the two rep-
resentations match as closely as possible, which may involve using third party
software. This process may be further complicated by the fact that different makes
of color printer can produce print-outs of the same screen image that look different.
Nowadays the flicker rate of most display screens is high enough for the flicker
to be imperceptible. There is some evidence, however, that the flicker rate can
affect how people read, because it changes the size of the saccadic eye movements
that take place during reading (e.g., see Kennedy and Baccino 1995 ). If you are
developing a system which requires people to read large amounts of text from the
display, you may want to make sure that you choose display screen hardware that
has a flicker rate that minimizes the amount of interference on reading.
If you are designing a system where you need the user to focus on one (or more)
particular items on a densely populated screen, you should consider whether you
can make those items pop-out from the display by appropriately changing some of
the items' features. There is a potential downside of working with densely pop-
ulated displays too, in that there may be some items which pop-out of the display
when you do not want them to. In these cases you would have to consider how you
could make those items appear more similar to the surrounding items.
4.5 Higher Level Visual Perception
In addition to how low level aspects of the eye influence perception, there are
several higher level aspects of vision that influence cognition. Several of the