There’s a common misconception about colour deficiency, and that’s what is called colour blindness. This misconception is understandable; after all, people with colour deficiency can’t see a certain range of colours. Colour blindness, however, should be reserved for people with a condition known as rod monochromacy, where colour distinction is severely impaired and the individual can only discern between black, white and shades of grey. To understand colour deficiency and colour blindness, it’s important to have a basic grasp of rods and cones.
Rods are responsible for vision in low-light environments and for peripheral vision, while cones, located towards the centre of the retina, are each sensitive to a particular wavelength of light. There is the short-wave cone, responsible for seeing blue light, the medium-wave cone, responsible for green light, and the long-wave cone, which enables us to see red light. Should one or more of these cones not function properly (or at all), you have a colour deficiency.
Red-green colour deficiency is found predominantly in males; that’s because the genes for the red and green cones are found on the X chromosome, so women would have to inherit two sets of defective chromosomes in order to be affected by these conditions. Protanomaly and protanopia are the two types which affect the long-wave cones; the first is a deficiency in the cones, while the second is their total non-activation. With Protanomaly, reds, oranges and yellows may appear more green, and colours may not be as bright. Protanopia results in reds looking like black; orange, yellow and green might all appear as yellow.
Deuteranomaly and deuteranopia follow the same convention; an abnormality in the green cones or their total non-activation. Greens and yellows appear more red, and violet becomes hard to distinguish from blue when you have deuteranomaly; deuteranopia means reds look brown and yellow and greens look beige.
The second broad category of colour deficiency is blue-yellow colour deficiency, which can affect males and females equally; it is also extremely rare. Tritanomaly is a deficiency in short-wave cones causing blues to appear more green. Tritanopia, total non-activation, makes yellows seem pink and blues look greenish.
When only one of the cones is active, the condition is referred to as monochromacy; ostensibly, you could “see” reds with red cone monochromacy, but what a colour really is when it’s indistinguishable from any other colour is an interesting philosophical question not to be answered here. Rod monochromacy occurs only when all of the cones are inactive, and you can only see through the rods; this condition is often coupled with an inability to see in bright light, as rods are meant to see in dim-light conditions.
There’s a wide variety of reasons for colour deficiency to develop, and they can be acquired even if you aren’t born with them. You can get an colour-testing eye exam to determine whether or not you’ve been affected by a colour deficiency; your optometrists can then offer you solutions to adapt to whatever deficiency you have.