All cells do different jobs. Because a cell is so tiny, it tendsto specialise in one role. Red blood cells job is to carry oxygen around the body. They have a red colouring because of an iron based compound called haemoglobin. When it has oxygen attached it goes red ( a bit like rust) and when it has given the oxygen to the cells it is blue (like in the veins on your wrist). Once it goes through the lungs it becomes red again. Because oxygen is so important red blood cells take up all there space with this process. They don’t even have a nucleus so they can’t do any other jobs. White blood cells are different. They have loads to do fighting infections and organising the immune system and don’t do any oxygen carrying so they aren’t brightly coloured.
As Sam said it is because red and white blood cells both do different jobs and they need to make sure they do it as well as possible to make sure we can survive.
Red blood cells are needed to carry oxygen to different parts of the body. They dont have a nucleas so they have space for the maximum amount of oxygen. White blood cells fight infections in the body and so they need a nucleas to act as their brain and tell the what to do and where to go.
If red blood cells tried to fight infections they would be pretty rubbish at it as they dont have a nucleas and white blood cells would be rubbish at carrying oxygen as they dont contain the important oxygen carrying molecule called haemoglobin.
Red blood cells like to keep up with the latest fashions, changing from red to blue outfits depending on how much oxygen they have, they are very snazzy! White blood cells act like an army in the body so they have to all wear the same white uniforms all the time!