It’s all to do with little cells at the back of your eye called ‘cone cells’ – these are responsible for allowing you to see in colour. There are three different types of cone cell and each type allows you to see a different colour – red, green or blue.
People who are colourblind have a genetic defect which means that one or more of these types of cell don’t work properly. It can also happen if your eye is damaged in some way (such as staring into a laser beam!) Red and green colourblindness is the most common, blue is quite rare. It’s 20 times as common in men as it is in women.
You can do a test to see whether you have colour blindness. Basically you look at pictures which have a bunch of coloured dots, where some of the dots trace out a number in a different colour. People who are colour blind have trouble seeing some of the numbers.