It comes as a total surprise to many people when they are found to have a deficiency or defect in colour recognition.  The frequent protestations are- I am perfectly healthy, I have had no eye problem ever, I can drive normally and have had no difficulty in my daily work so far!  The list is endless.

Colour perception is unique to primates and most developed in Humans.  We are able to differentiate between various wavelengths in the visual spectrum and have given them specific names as well. The rainbow spells our classification violet, indigo, blue, green, yellow, orange and red (VIBGYOR). There are innumerable hues in between the different colours that we perceive and enjoy the beauty of nature.  We do not perceive wavelengths outside the visual spectrum.

Light and Dark perception is universal in animal kingdom.  

While vision is a complex psycho-optical function, it starts at the retina of the eye from special cells that receive light and process it. They are called photo receptors and are characterised by unique shapes- Rods ( for light perception) and Cones (for colour perception).  There are cones are specific to each primary colour-Red, Green, Blue primarily.

Defect in the function of cones related to a specific colour create colour deficiency or defect popularly called 'Colour Blindness'. Colour Blindness is a genetically transmitted character and is linked to female chromosome in a recessive way.  Women carry the defect but it is the males who are affected.  Rarely a woman can also be affected but the probability is very low. A man found to have colour blindness may have male cousins from mother's side affected as well.

There is no problem in routine functions as colour identification is a learnt behaviour. They identify but do not perceive the way a normal person does. It is unmasked while doing complex tasks requiring matching of colours and hues and sorting coloured objects.  A person may never be diagnosed of colour deficiency, though defects are easier to catch.

No person can 'learn' or 'practice' to pass a colour deficiency test.  The most commonly used screening test is ISHIHARA charts that come in a booklet.  Many people try to remember page number or practice a numeral that is seen to a normal person, well it is of little use.  a doctor will check pages randomly and the patterns are different in different books. Test are carried out at multiple levels and increasing sensitivity beyond the charts for specific jobs and fluke chance may not favour again.

There are so many jobs that do not have colour vision as a mandatory physical fitness parameter. 

There is no reason for despair as you can choose a profession and achieve great heights by putting in hard work.