@222heaa23 Blood is red because it is mainly made up of red blood cells which contain haemoglobin a molecule that carries oxygen around the body. Haemoglobin contains iron (Fe). When iron is exposed to oxygen it turns reddish what we see as rust in metal. When haemoglobin in the red cell binds oxygen molecules it turns bright red. When it releases the oxygen molecules it turns dark red. Haemoglobin binds oxygen in the lungs and is pumped by the heart throughout the body via arteries circulating oxygen. In the tissues it releases oxygen and travels back to the lungs via veins.
Note that veins appear blue or green in colour. This is not because blood in the body is green but because of the colour of the skin. Imagine looking at red through a yellow-brownish translucent material. The colour you will see is not red but a mixture of red and the colour of the medium you are looking through. In our case, brown skin, blood in the veins appears green.