Webstandards, Accessibility and Usability at Penn State

This Page

1. Why Red/Black Can be Bad
2. Why Red/Green Can be Bad
3. Merry Christmas in Different Color Vision Modes
4. Tests for Simulating Color Blindness
5. Color Blindness Links

The human eye contains rods which detects levels of light (dark versus bright) and three kinds of cones which distinguish among the different colors. If none of the cones are functioning (very rare), then a person has grayscale vision. If the cones are semi-functional (also rare), then the person sees colors, but they are muted.

More commonly, a person with color blindness (or "color deficiency") has a difference in only one of the rods, and it is usually either the red rod or the green rod. The result is that some colors appear to be more similar or are muted. The different type of deficiencies are simulated below.

Why Red/Black Can Be Bad

Some color blind users are lacking the capability to detect the lower color wave frequencies associated with red. For these users, red color waves read as "no signal", or "black". These users confuse red and black, so this contrast should be avoided whenever possible. Red and white is legible, but indistinguisable from black and white.

Sample Warning Signs

The black text on red sign becomes black on black for some color blind users.

Why Red/Green Can be Bad

If one or two sets of cones do not function correctly, then a person will have trouble telling certain colors apart. Almost 10% of men are red/green color blind; another group are blue/yellow color blind. Despite the fact that red-green contrasts are very distinct for about 95% of humanity, there are about 5% of people for whom this is non-functional. This is exacerbated by the fact that red and green are nearly identical on a gray scale monitor. See examples below.

NOTE: Red-Green color blind people are better able to find camouflaged objects in natural settings. See Visicheck Article for more details.

Merry Christmas in Different Color Vision Modes

The paragraph above is "Merry Christmas" in green text on a red background which a color blind person may not be able to read. This is also problematic because it causes a visual vibration for users with normal color vision.

In Grayscale

In grayscale, there is only a slight contrast because red and green are of nearly equal brightness.

In Deuterope Mode (Red/Green Color Blindness)

Appears as dark olive on medium olive in normal color vision. Simulation courtesy of Visicheck.

In Tritanope Mode (Blue/Yellow Color Blindness)

Actually appears as teal and magenta. Simulation courtesy of Visicheck.

More Accessible "Merry Christmas" Banner with Yellow Background

The yellow Background separates the areas of red and green and provides a contrast in brightness, making the sign more legible. See the grayscale example below.

In Grayscale

Tests for Simulating Color Blindness

Although it is difficult to determine exactly how a color-blind person sees the world, it is usually the case that if something is legible in grayscale, then a color-blind person can also view the information, although it may not be particularly appealing to that person. This is because, in most cases, the rods are functional and are still able to distinguish levels of light and dark.

Note: There are some color combinations, such as some combinations of teal and yellow which are O.K. in grayscale, but still problematic for color blind users.

Some Color Blindness Simulators

1. Color Vision (Cal Henderson) - Test color schemes with almost all forms of color blindness

2. Visicheck Color Blindness Tester - Allows you to test images and live Web pages for red-green and blue-yellow. color deficiencies.

Color Blindness Links

Color Blindness Simulators

• Wickline Color Filter - Lets you toggle between different forms of color blindness once you designate a Web address.
• Visicheck Color Blindness Tester - Allows you to test images and live Web pages for red-green and blue yellow color deficiencies.
• Color Scheme Designer & Color Blindness Checker - Generates basic color schemes and shows them in different color blindness filters below color blocks
• Color Vision (Cal Henderson) - Test color schemes with almost all forms of color blindness
• Hans Brettel - Examples only

About Color Blindness

• Lighthouse Org Color Contrast - General tips on how to combine colors.
• University of Minnesota Duluth Color Links - Includes a variety of color blindness resources.
• Joe Clark - A very accessible description of the different types of color blindness.
• Colors for the Color Blind - An informative yet wry look at color blindness from an actual "user". Includes links to utilities for the color blind.
• Firelily
• WebAIM Color Blindness
• Visibone - Scroll down for color blindness information
• Colblinder Blog - from a color blind writer and color blindness expert

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