Robin Shannon wrote: Our links are blue and red. Can you confirm that these really are indistinguishable to someone with red-green colorblindness? I would really like to know if this is based on actual experience (eg, you are colorblind or a colorblind person has reported to you that they are in fact not distinguishable) or if it's speculation. Without hard data it's going to be very difficult to just come up with a change. -- brion vibber (brion @ pobox.com)

Dear all, I thought I could help cast some light on this for those interested, both from a medical point of view and from a colour blind person's view. There are many types of colour blindness. The eye as you probably know have two different light perceiving cells, the rod cells and the cone cells. The rods are very light sensitive but can only detect light in a certain frequency range (peak at 505 nm), and it is interpreted in our brain as shades of grey. The less light sensitive cone cells are what we use for our daily colour perception. There are normally three types of cone cells, with different sensitivities to different wavelengths, viz. 445 nm (blue, greek prefix trit-), 535 nm (~green, deuter-), and 570 nm (~red, prot-). Combined, these can produce an image interpreted by our brain as the different colours we know. In colour blindness, people have either a defect or a disproportionally low number of one of the cones (-anomaly) or they have a complete (functional or real) absence of one of the cones (-anopia). In order of frequency the types of colour blindness are rougly deuteranomaly (~6% of males), protanomaly, deuteranopia and protanopia (each ~1%) and tritanomaly and tritanopia (both very rare). Very few people have no colour vision at all. The common types of colour blindness are inherited on the X-chromosomes which means that predominantly men are affected, but about 1% of colour blind are still women. This happens when women inherit two affected X-chromosomes. Because the green and red colour blind (deuter- and protanomalous/-anopic) have defects in a similar frequency range (535 vs 570 nm), the clinical effects are similar, although they may be leaning towards problems in either green or red perception, respectively. In addition, this group will in some circumstances have difficulties discerning the two colours when mixed. Myself, I am deuteranomalous. When people ask, I usually describe the green that I see as "less green" or "a greyer shade of green", but I wouldn't really know as I've never known any different. Driving at night, I can easily tell a red light, but I may sometimes confuse a green light with a white light, thinking it is an approaching car or a street light. So I take extra care when driving in the city, looking at the shape of the light when in doubt and the context. The difficulty in perceiving colour applies especially if we're talking about thin lines or writing. I've been known to be writing with a green pen thinking it was a weak black pen. I also have problems distinguishing red and green when mixed, such as green writing on red background or vice versa, or, as a kid, picking strawberries :) In conclusion, the links as they are now on Wikipedia are as good as they can be for me personally as a deuteranomalous person. I can, like most people, easily tell what's blue, and personally I perceive red quite well, and have no problem seeing what's purple. However, there may be people who have a less intense perception of the red. They would still, however, be able to distinguish it from blue, and probably even from purple. On 26/06/05, Anthere <anthere9(a)yahoo.com> wrote: Stubs are purple red? I haven't noticed :D Can you give me a link to a page which contain examples of all these four coloured links? So based on what I said above, I think the current colour coding is actually if anything optimised for the majority of colour blind people, as long as we don't also throw in a green link somewhere. Cheers, Bjarte Sørensen http://meta.wikimedia.org/wiki/User:BjarteSorensen