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GUIDE LINES FOR EFFECTIVE COLOR TERMINAL USAGE
Based on research done at Tektronix
and reported in Handshake, 1985.
PHYSIOLOGICAL GUIDELINES:
Avoid simultaneous display of highly saturated, spectrally extreme
colors. Reds, oranges, yellows, and greens can be viewed without refocusing,
but cyan and blues cannot be easily viewed at the same time at as red. To
avoid frequent refocusing and visual fatigue, extreme color pairs such as red
and blue or yellow and purple should avoided. However, desaturating
spectrally extreme colors will reduce the need for refocusing.
Pure blue should be avoided for text, thin lines, and small shapes.
Our visual system is just not set up for detailed, sharp short-wavelength
stimuli. However, blue does make a good background color and is perceived
clearly out into the periphery of the visual field. In fact, due to the eye's
reduced sensitivity to blue, the raster scan pattern in a blue background
display is less visible then when other background colors are used.
Avoid adjacent colors differing only in the amount of blue. Edges
differing only in the amount of blue appear indistinct.
Older viewers need higher brightness levels to distinguish colors.
Colors change appearance as ambient light level changes. Displays
change color under different kinds of light - fluorescent, incandescent, or
daylight. Appearance also changes as the light level is increased or
decreased. On the one hand, a change due to an increase or decrease in
contrast occurs, and on the other, due to the shift in the sensitivity of the
eye.
The magnitude of a detectable change in color varies across the
spectrum. Small changes in extreme reds and purples are more difficult to
detect than changes in other colors such as yellow and blue-green. Also, our
visual system does not readily perceive changes in green.
Difficulty in focusing results from edges created by color alone. Our
visual system depends on a brightness difference at an edge to effect clear
focusing. Multi-colored images then, should be differentiated on the basis of
brightness as well as color.
Avoid red and green in the periphery of large scale displays. Due to
the insensitivity of the retinal periphery to red and green, these colors in
saturated form should be avoided, especially for small symbols and shapes.
Yellow and blue make good peripheral colors.
Opponent colors go well together. Red and green or yellow and blue are
good combinations for simple displays. The opposite combinations - red with
yellow or green with blue - produce poorer images.
For color deficient observers, avoid single-color distinctions.
PERCEPTUAL GUIDELINES:
Not all colors are equally discernible. Perceptually, we need a large
change in wavelength to perceive and identify a color difference in some
portions of the spectrum and a small one in other portions.
Luminance does not equal brightness. Two equal luminance colors will
probably appear to have different brightnesses. The deviations are most
extreme for colors towards the ends of the spectrum (red, magenta, blue).
Different hues have inherently different saturation levels. Yellow in
particular always appears less saturated than other hues.
Lightness and brightness are confounded on a color display, but not on
a printed hard copy. The nature of a display does not allow lightness and
brightness to be varied independently.
Not all colors are equally readable or legible. Extreme care should
be exercised with text color relative to background colors. Besides a loss in
hue with reduced size, inadequate contrast frequently results when the
background and test color are similar. As a general rule, the darker,
spectrally extreme colors such as red, blue, magenta, brown, etc., make good
backgrounds while the brighter, spectrum center and desaturated hues produce
more legible test.
Hues change with intensity and background color. In grouping elements
on the basis of color, be sure that backgrounds or nearby colors do not change
the hue of an element in the group. Limiting the number of colors and making
sure they are widely separated in the spectrum will reduce confusion.
Avoid the need for color discrimination in small areas. Hue
information is lost for small areas. Hue information is lost for small
areas. In general, two adjacent lines of a single pixel width will merge to
produce a mixture of the two. Also, the human visual system produces sharper
images with achromatic colors. Thus, for fine detail it's best to use black,
white, and gray while reserving chromatic color for larger panels or
attracting attention.
COGNITIVE GUIDELINES:
Don't overuse color. Perhaps the best rule is to use color sparingly.
The benefits of color as an attention getter, information grouper, and value
assigner are lost if too many colors are used. Cognitive scientists have
shown that the human mind experiences great difficulty in maintaining more
than 5 to 7 elements simultaneously. So it's best to limit displays to about 6
clearly discriminable colors.
Be aware of the nonlimear color manipulation in video and hard copy.
At this time algorithms do not exist for translating the physical colors of an
imaging device into a perceptually structured color set. Video or hard copy
systems cannot match the human perception and expectations on all fronts.
Group related elements by common background color. Cognitive science
has advanced the notion of set and preattentive processing. In this context,
you can prepare or set the user for related events by using a common color
code. A successive set of images can be shown to be related by using the same
background color.
Similar colors connote similar meaning. Elements related in some way
can convey that message through the degree in similarity of hue. The color
range from blue to green is experienced as more similar that the gamut from
red to green. Along the same lines, saturation level can also be used to
connote strength of the relationship.
Brightness and saturation draw attention. The brightest and most
highly saturated area of a color display immediately draws the viewer's
attention.
Link the degree of color change to event magnitude. As an alternative
to bar charts or tic marks on amplitude scales, displays can portray magnitude
changes with the progressive steps of changing color. A desaturated cyan can
be increased in saturation as the graphed elements increase in value.
Progressively switching from one hue to another can be used to indicate
passing critical levels.
Order colors by their spectral position. To increase the number of
colors on a a display requires imposing a meaningful order on the colors. The
most obvious order is that provided by the spectrum with the mnemonic ROY G
BIV (red, orange, yellow, green, blue, indigo, violet).
Warm and cold colors should indicate action levels. Traditionally,
the warm (long wavelength) colors are used to signify action or the
requirement of response. Cool colors, on the other hand, indicate status or
background information. Most people also experience warm colors advancing
towards them - hence forcing attention - and cool colors receding or drawing
away.
While these guidelines offer some suggestions, they certainly
shouldn't be taken as binding under all circumstances. There are too many
variables in color display, color copying, human perception, and human
interpretation to make any hard and fast roules. So, by all means, experiment.