Colour-blindness – When colours cannot be seen

When the colours of the spectrum are arranged on a palette, a person with normal vision is presented with the whole colour spectrum. However, for someone suffering from red-green colour-blindness, the palette has only two colours: blue in the cold area and yellow in the warm area. In between he perceives hardly any colour, mostly just white or grey. So if he wants to paint using a blueish green – a colour which lies in the grey area between the warm and cold colours – he swings between purple-red, grey and blue-green, which all appear alike. In the same way he will be unable to differentiate between yellow, green and orange, and likewise blue and crimson.

In Europe approximately eight percent of men, but only 0.5 percent of women suffer from a more or less pronounced red-green weakness. Men are more greatly affected, because this defect is passed on via the X chromosome. As men, in contrast to women, possess only one X chromosome, defective genes in this chromosome cannot be compensated by the healthy gene(s) of the second X chromosome as happens with women.

A person with normal vision can easily simulate colour-blindness by looking at the world through a powerful coloured filter – such as a piece of coloured glass. But changes in light conditions also influence the colour perception of people with normal vision: when it slowly becomes dark at night, red and green shades are the first to fade, and in the end it is no longer possible to see any colours at all. The reason for this is that the colour sensitive cells of the eye (the cones) need strong light to function. The second type of light sensitive cells (the rods), which only perceive differences in light intensity but no colours are already active in very weak light. The result is that in twilight or moonlight the world no longer appears in colour but in shades of grey. Hence the popular proverb, „All cats are grey at night“.