A client emphasised an important design point during a meeting yesterday when he told us he was colour blind and couldn’t distinguish between the colours red and green. He said this can often lead to difficulties reading text on websites. And our client is not alone.

Colour blindness actually affects nearly 10% of the male population (but only about 0.4% of the female population). This means that 1 in 10 of the males who use the internet are seeing websites with a completely different colour scheme to that intended by the designer.

Colour blindness is a condition in which the individuals affected have a partial or total inability to detect certain wavelengths of the visual spectrum. It is typically genetic in nature but may also occur because of eye, nerve, or brain damage, or due to exposure to certain chemicals. The total inability to see colour is extremely rare and so the disorder should really be known as colour vision deficiency or colour defective vision.

Colour blindness varies between individuals in both the insensitivity and the wavelengths they are unable to see. The most common defect involves the green receptors and leaves people unable to distinguish red and green, but still sensitive to red light. The next most common failure is in the red receptors. These people also confuse red and green but are insensitive to red light.

The effects of red-green colour blindness can be described as follows:

• Protanopia (red deficiency) - blue-green appears grey, red-purple appears grey. This is a rare form of colour blindness.

• Protanomalia (partial red deficiency) - blue-green appears indistinctly greyish, red-purple appears indistinctly greyish. This is also rare, affecting about 1% of the male population.

• Deuteranopia (green deficiency) - green appears grey, purple-red appears grey. This also affects about 1% of the male population.

• Deuteranomalia (partial green deficiency) - green appears indistinctly greyish, purple-red appears indistinctly greyish. This is the most common form of colour blindness and affects about 8% of the male population.

Luckily there is help at hand when designing websites to ensure they are “colour blind friendly”. The ColorBlind Webpage Filter helpfully shows how your webpage looks under the various forms of colour blindness as described above.

By Calum Land on Wednesday, 6th September, 2006 at 4:21 pm