When people hear the term “colour-blind”, they typically connect it to the words colour blindness, which is incorrect. As confusing as it may sound, being colour blind does not necessarily mean devoid of seeing colour. Instead, colour-blindness is just a general word for someone who has a colour visual impairment, meaning that they see fewer colours than normal.

A normal person has three different cone cells in their eyes. These send specific signals when they react to specific colours. Someone who is colour-blind has a problem with their different cone types, which means that they may be unable to see some colours, but not all.

Now, monochromacy is a super rare form of complete colour-blindness where the cone cells themselves fail to function. There are a few different types, though the far most common is rod monochromacy, or achromatopsia. This is the complete inability to distinguish between colours. If cone cells are sensitive to colour, then rod cells are sensitive to light. Someone with achromatopsia have non-functioning cone cells and perceive the world entirely in grey.