Five to ten percent of all people have a deficiency in color vision
known as “color blindness”. Most people can see colors to some
degree, but not all people see colors in the same way. For example,
people with Protan vision have difficulty seeing red, people with
Deutan vision have difficulty seeing green, and people with Tritan
vision have difficulty seeing blue. Avoid using certain color
combinations in critical applications, because not all people can
distinguish them. For such color blind users, you must use what is
known as “color picking”, which is the
deliberate avoidance of colors that color blind users are unable to
distinguish. Specifically, when you use a set of colors in a
diagram or chart, you should avoid using red and green in the same
display for red-green-blind users, but you can use a range of
colors from blue to yellow. The same applies to using blue and
yellow in displays for blue-yellow-blind users.
In accessible applications, the information that is presented by
using colors is also presented in other ways. The colors that are
used therefore become redundant and are known as “redundant colors”. For example, the data
presented in a diagram might be presented additionally as text in a
table. Graphic objects might be presented additionally as specific
shapes, or with alternative color schemes, so that colors are not
the only way to distinguish items.
To provide alternative color schemes for
color blind users, you can specify color combinations through
cascading style sheets (CSS). The cascading feature makes it
possible to combine different CSS files.
For example, you can use a combination of:
- A CSS file that specifies the structure and dimensions of graphic objects.
- A CSS file that specifies alternative shapes for graphic objects.
- A CSS file that specifies colors.
You can select the CSS file that specifies
colors from three variants:
- A variant for normal users.
- A variant for red-green blind users.
- A variant for blue-yellow blind users.
For dashboard diagrams, you can have
different versions of symbol palettes and select the appropriate
version.