One of the important things to consider while studying web accessibility is the colors used in a web page. We usually talk about color blindness but sinceraly I don't know how does it really look. Color blindness is typically a genetic condition, and it is much more common in men than in women. Approximately one in 12 men has at least some color perception problems. Less common, acquired deficiencies stem from injury, disease, or the aging process. Also, although not called "color blindness," when people age, their corneas typically turn yellowish, severely hampering their ability to see violet and blue colors.

I found the previous figure an excellent example showing the different kind of color blindness, what does the image really look and how do people see it (Protan, Deutan and Tritan).

People with normal cones and light sensitive pigment (trichromasy) are able to see all the different colors and subtle mixtures of them by using cones sensitive to one of three wavelength of light - red, green, and blue. A mild color deficiency is present when one or more of the three cones light sensitive pigments are not quite right and their peak sensitivity is shifted (anomalous trichromasy - includes protanomaly and deuteranomaly).