Your eyes contain special cell structures to see light. Long skinny cells see black and white which is commonly called monochrome. Cone shaped cells see colour. Anything that damages the cone cells can make you colour blind.
Imaging a table covered in carrots pointing toward the ceiling. Those are the cone cells that see colour. Some of the cone cells see red, some see green, and some see blue. Imagine the table covered in red, green, and blue coloured carrots.
Now imagine pencils scattered among the carrots. The pencils are also pointing upward. The pencils are the rod cells that see monochrome. There are many cone cells and few rods in the centre of the table. As you move out toward the edge of the table, the cone cells decrease and their space is taken by rods. At the edge of the table there are only rods.
Genetic disorders can make your eye different. Rod cells developed before cone cells and are a real advantage in the dark jungle because rod cells see in lower levels of light. We need rods and can live without cones. The most common genetic disorder in eyes is to have less developed cone cells.
You can also have disease damage and destroy the function of cells in your eye. Get your eyes checked regularly. The cone cells are more complex so may be damaged before the rod cells. If you start having problems seeing colour then it is the first symptom of some diseases especially diabetes, a disease that is becoming more common because of junk food and advertising of junk products to children on television.
Deuteranopia is the fancy name for colour blindness where you cannot see red or green. Eight percent of European males have Deuteranopia which means they cannot tell if traffic lights are red or green. Half of once percent of European women have the same problem so both men and women should have their eyes carefully checked before getting a drivers licence. Deuteranopia is less common in other parts of the world. Deuteranopia is caused by a gene that does not work. You end up with eyes that are missing the cone cells for red and the cone cells for green.
Protanopia is the formal name for colour blindness where you are missing the blue cones. You cannot see the blue in blueberries or the blue tongue of the Blue Tongue Lizard. Protanopia is very rare and you can live without seeing the blue tongue of the Blue Tongue Lizard. If you start having problems seeing blue then have your blood tested for diabetes mellitus and liver disease.
Achromatopsia is an extremely rare form of colour blindness where you have lots of rod cells and no cones. The rods are very sensitive to light which means people with Achromatopsia have to stay in doors all day or wear sunglasses. Perhaps vampires have Achromatopsia.
The codes and rods are in the retina of your eye. Anything that damages the retina or the blood supply to the retina can cause loss of vision including colour distortion and colour blindness. Chemicals including drugs, medicine, and industrial chemicals can cause colour blindness. Degenerative diseases, particularly those that damage your blood supply or nerves, can produce colour blindness. When you stop seeing red or green or blue, go to the doctor.
When you are really old you might have trouble seeing colour because your eyes are just plain worn out. Get a complete physical check up before giving up on seeing colour.