I don’t know. There could be several reasons, and several explanations. One explanation might be ‘Because you are male’ – colour blindness is very rare in females. The most common type of colour-blindness is red-green colour blindness.
In normal colour vision, light stimulates three different types of cells in the retina of our eyes. We generally call these cells ‘red’, ‘green’ and ‘blue’ but in fact they all respond to a wide range of wavelengths of light. Our sensation of colour depends on the relative stimulation from these cells. The graph below shows ‘sensitivity of the three types of cone cells to light of different frequency.
If we ignore the blue cells for the moment then:
- If the red cells are stimulated more than the green cells then we have a sensation we call ‘Red’
- If the red cells are stimulated about the same as the green cells then we have a sensation we call ‘Yellow’
- If the red cells are stimulated less than the green cells then we have a sensation we call ‘Green’
What you notice is that the red and green cells have a very similar response. In people who are colour blind, the response curves may be more similar giving them weaker ability to distinguish between shades of red and green and yellow. In some people one type of either red or green cell is completely absent and these people cannot distinguish reds and greens at all.