An object’s colour is our perception of the wavelength of light that bounces off the object. The eye captures these "bounces" with different wavelengths through the retina’s cones, each of which is connected to the visual centre of the brain via the optic nerve. The cones have three types of photopigment: one that is especially sensitive to red light, one to green light and a third to blue light. The combination of these three colours allows us to see about 20 million different colours. For example, orange is yellow combined with red or violet, red with a little blue. This interpretation is carried out in the brain. Colour-blind people are unable to distinguish colours well due to a failure of the genes that are responsible for producing cone pigments. If the faulty pigment is red, the person will not be able to distinguish red or its combinations. Colour blindness can also occur when one of the cone types is missing, known as dichromacy, when there are two, and monochromacy, when there is only one. The genetic defect in colour-blind people is inherited and is transmitted by a recessive allele linked to an X chromosome. If a male inherits an X chromosome with this deficiency, he will be colour blind. In the case of women, however, they will only be colour blind if their two X chromosomes have the deficiency. If not, they will only be carriers and may transmit it to their children. As a result, there is a larger predominance of men among those affected.
How can it be diagnosed?
As the disruption of vision in colour-blind people is small, the majority of people with this defect are unaware that they have it. To diagnose or evaluate the type and degree of severity, a number of subjective screening tests can be used to obtain quick results, such as the Ishihara colour test. This test uses a series of cards with various circles of different sizes and colours, with numbers inside that can only be identified by someone with good colour perception. When the test is used with small children, the numbers are replaced with designs or geometrical shapes. Other tests require more time, such as the Farnsworth-Munsell, in which the test subject orders a scale of different shades of colour in the same range. To find out if children have difficulty learning and distinguishing colours, the way they colour in pictures can be observed. Colour blindness in school-age children can cause learning disabilities when, for example, images, graphs and tables are used that can be difficult to understand for children with colour blindness. It is, therefore, important that parents are aware of the problems their children might have with colour vision and, if so, inform the school at the beginning of the school year. In the case of adults, colour blindness can cause problems in their professional lives. For example, people with the condition cannot join the police force or fire brigade, or be pilots, ships’ captains, etc.