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Making a site accessible to all means among other things the use of contrast and color intensities that do not interfere with the readability of it.

Accessibility has not been created for the sole benefit of the blind, as some might believe, but that of all disabilities (whether visual or other), and regardless of the degree of disability in question . By definition, is the accessibility of allowing everyone access to a resource, indiscriminate physical or hardware / software.

The problem

Visual disabilities are the most obvious, but the blind are not the only ones to benefit from more attention: the visually impaired and colorblind also form a significant portion (if not increasing with the aging of the population) users. If we combine the various possible visual impairment, you will realize that those who have perfect vision do not form such a large majority - and yet it is too often alone sites are built.
Choosing the right color combinations can easily provide better readability, and therefore better accessibility for the visually impaired. Among the visually impaired are people with blindness, that is to say, a poor interpretation of colors.

Definitions

Where normal vision combines three pigments (red, green and blue) to interpret the colors, the colorblind can:

• have three, one or more with Disabilities: This is abnormal chromaticism with anomalously for red, green and Deuteronomy for trigonometrical for blue.
• run out of the three: the chromaticism with Deuteronomy for red and blue propitiation for green and blue, and Mauritanian for red and green.
• lacking in two of the three: the monochromatic, the person sees the world in shades of black and white or with colors very washed out.

Potential

The first point to check, nothing in the web interface should be based solely on the color: no alert message with a red one for differentiation, for example. It is necessary that information can be understood differently: placement, context, markup, texture, indicators ...
The second point and ensure a good readability in the working color contrasts used. These contrasts play on the intensity of the color and the difference between two colors. The evaluation of the correct dosage may be too subjective, the W3C has proposed an algorithm for each, and some online tools exist to test their choice of colors and contrasts VischeckURL , Color Scheme Generator , Colour Contrast Check or Color vision . It will ensure that the difference in intensity between the relevant elements (texts) and those not relevant (colored background) is obvious.
Avoid finally combine bright colors bright end of the spectrum (red, blue) with dark colors in the middle of the spectrum (yellow, green), because the extremes appear darker than those who have disabilities, which reduces the contrast possible.

We therefore seek to combine bright colors in the middle of the spectrum with the dark colors of the extremities.