Color disabilities are often not taken seriously because mild cases are quite frequent. Although many cases are mild there are color disabilities that can render many web sites completely inaccessible.

There are two types of photoreceptors in the eye: rods and cones. Rods, which provide vision in dim light, have no ability to distinguish between colors. Cones are responsible for color vision. Color vision deficiencies result from either a lack of one or more cone types, or cones that behave differently from average.

One who has Rod monochromic or congenital achromatopsia is totally colorblind or almost totally colorblind. These people also have poor visual acuity. This phenomenon is rare. Without normal cone vision, their eyes are not able to adapt normally to higher levels of illumination.

There are many variations in the severity of these symptoms. There are complete rod monochromats, incomplete rod monochromats, and blue cone monochromats. Complete rod monochromats have the most severely impaired vision of all achromats. These people cannot distinguish any hues (e.g., blue, green, yellow and red.) They also have poor visual acuity, aversion to bright light and nystagmus (an involuntary, rapid movement of the eyes).

Cerebral achromatopsia (unlike other achromatopsia ) report that they see a monochromatic world, all in shades of gray (not accompanied by severely impaired vision, extreme light sensitivity, or any abnormality in the photoreceptors of the retina).

The Achromatopsia Network has told me that their members frequently report having trouble seeing the information on web pages because of the colors chosen for graphics, text and background, etc. It is important to have a strong ocular contrast. In this case, they do not perceive color at all. Certain shades of one color will be indistinguishable to them from certain shades of another color in the background -- thus, they may be unable to read or perceive some material at all if the background color in gray-scale looks like the foreground (or print) color in gray-scale. The biggest problem in common usage is black on red or red on black, since black and red look the same to their eyes.

And now to the point - maintain high contrast on a gray black - white in all color schemes. (Especially where control is not handed over to the user.) This can easily be tested on a black and white monitor.

Dichromacy is a less severe form of color defect than monochromacy. Dichromats can tell some hues apart and it is much more common. Dichromacy is divided into three types: protanopia, deuteranopia and tritanopia.

Protanopia and deuteranopia are red-green defects. Persons with red-green defects have difficulty distinguishing between reds, greens and yellows but can discriminate between blues and yellows. Protanopes often can name red and green correctly because green looks lighter to them than red.

Hereditary tritanopia is a blue-yellow defect. Persons with blue-yellow defects cannot see the difference between blues and yellows but can distinguish between reds and greens. (Less common.)

Anomalous Trichromacy-The ability of anomalous trichromats to distinguish between hues is better than dichromats but still not normal. Red-green anomalous trichromacy is subdivided into protanomaly and deuteranomaly.

A third type of anomalous trichromacy is tritanomaly. In fact, those suffering from any of these conditions do experience color, but not in the sense that a color "normal" observer does.

Some sites (seemingly reputable) claim that with color deficits, ability to discriminate colors on the basis of all three attributes - hue, lightness and saturation - is reduced. Designers can help to compensate for these deficits by making colors differ more dramatically in all three attributes.

So, for Dichromacy avoid the following color combinations:

• Red-green
• blues and yellows
• reds, greens and yellows

Using colors that mix a blue with a red and a green with a yellow would work.

Alternatively, you can hand control over to the user so that all text and semantic information is presented as text. They can then link to their own style sheet that works for them. Of course that would not work for graphics, unless SVG (scalar vector graphics) or a similar graphic markup language is used.

All of the above, of course, no automated tool can tell you (like Bobby) - unless you complete the additional checks by hand.