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Physiological
guidelines
Avoid
the simultaneous display of highly saturated, spectrally
extreme colours. Reds, oranges, yellows, and greens can
be viewed together without refocusing, but cyan and blues
cannot be easily viewed simultaneously with red. To avoid
frequent refocusing and visual fatigue, extreme colour
pairs such as red and blue or yellow and purple should be
avoided. However, desaturating spectrally extreme colours
will reduce the need for refocusing.
Avoid
pure blue 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 colour and is perceived clearly out into the
periphery of our visual field.
Avoid
adjacent colours differencing only in the amount of blue.
Edges that differ only in the amount of blue will appear
indistinct.
Older
viewers need higher brightness levels to distinguish
colours.
Colours
change appearance as ambient light level changes.
Displays change colour under different kinds of ambient
fight - Fluorescent, incandescent, or daylight.
Appearance also changes as the light level is increased
or decreased. On the one hand, a change occurs due to
increased or decreased contrast, and on the other, due to
the shift in the sensitivity of the eye.
The
magnitude of a detectable change in colour varies across
the spectrum. Small changes in extreme reds and purples
are more difficult to detect than small changes in other
colours 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 colour alone.
Our visual system depends on a brightness difference at
an edge to effect clear focusing. Multi-coloured images,
then should be differentiated on the basis of brightness
as well as of colour.
Avoid
red and green in the periphery of large-scale displays
Due to the insensitivity of the retinal periphery to red
and green, these colours in saturated form should be
avoided, especially for small symbols and shapes. Yellow
and blue are good peripheral colours.
Opponent
colours go well together. Red and green or yellow and
blue are good combinations- for simple displays. The
opposite combination - red with yellow or green with blue
- produce poorer images.
For
colour deficient observers, avoid single colour
distinctions.
Perceptual
Guidelines
Not
all colours are equally discernible. Perceptually, we
need a large change in wavelength to perceive a colour
difference in some portions of the spectrum and a small
one in other portions.
Luminance
does not equal brightness. Two equal luminance but
different hue colours will probably appear to have
different brightness. The deviations are most extreme for
colours towards the ends of the spectrum (red, magenta,
blue).
Different
hues have inherently different saturation levels. Yellow
in particular always appears to be less saturated than
other hues.
Lightness
and brightness are distinguishable on a printed hard
copy, but not on a colour display. The nature of a colour
display does not allow lightness and brightness to be
varied independently.
Not
all colours are equally readable or legible. Extreme care
should be exercised with text colour relative to
background colours. Besides a loss in hue with reduced
size, inadequate contrast frequently results when the
background and text colours are similar. As a general
rule, the darker, spectrally extreme colours such as red,
blue, magenta, brown, make good backgrounds while the
brighter, spectrum-centered, and desaturated hues produce
more legible text.
Hues
change with intensity and background colour. When
grouping elements on the basis of colour, be sure that
backgrounds or nearby colours do not change the hue of an
element in the group. Limit the number of colours and
making sure they are widely separate in the spectrum will
reduce confusion.
Avoid
the need for colour discrimination in 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 colours.
Thus for the fine detail, it is best to use black, white,
and grey while reserving chromatic colours for larger
panels or for attracting attention.
Cognitive
Guidelines
Do
not overuse colour. Perhaps the best rule is to use
colour sparingly. The benefits of colour as an attention
getter, information grouper, and value assigner are lost
if too many colours are used. Cognitive scientists have
shown that the human mind experiences great difficulty in
maintaining more than five to seven elements
simultaneously so it is best to limit displays to about
six clearly discernible colours.
Be
aware of the nonlinear colour manipulation in video and
hard copy. At this point, algorithms do not exist for
translating the physical colours of an imaging device
into a perceptually structured colour set. Video or
hard-copy systems cannot match human perception and
expectations on all fronts.
Group
related elements by using a common background colour.
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
colour code. A successive set of images can be shown to
be related by using the same background colour.
Similar
colours connote similar meanings. Elements related in
some way can convey the message through the degree in
similarity of hue. The colour range from blue to green is
experienced as more similar than the gamut from red to
green. Along these lines, saturation level can also be
used to connote the strength of relationships.
Brightness
and saturation draw attention. The brightest and most
highly saturated area of colour display immediately draws
the viewer's attention. Link the degree of colour change
to event magnitude. As an alternative to bar charts or
tic marks on amplitude scales, displays can portray
magnitude changes with progressive steps of changing
colour. 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
colours by their spectral position. To increase the
number of colours on a display requires imposing a
meaningful order on the colours. 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 colours should indicate action levels.
Traditionally, the warm (long wavelength) colours are
used to signify action or the requirement of a response.
Cool colours, on the other hand, indicate status or
background information. Most people also experience warm
colours advancing toward them - hence forcing attention -
and cool colours receding or drawing away.
While
these guidelines offer some suggestions, they certainly
should not be taken as binding under all circumstances There
are too many variables in colour display, colour copying,
human perception, and human interpretation to make any hard
and fast rules. So, by all means, experiment.
back
to Color
Murch, G. (1995). Color graphics -
Blessing or Ballyhoo (Excerpt). In Baecker, R. M., Buxton,
W., & J. Grudin (Eds.) Readings in Human-Computer
Interaction: Toward the Year 2000. San Francisco, CA:
Morgan Kaufmann.
Original publication in Computer
Graphics Forum. Permission to reproduce on the web granted
by Eurographics: The European Association for Computer
Graphics.
back to referring page:
Color
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