The red colour of RBC is due to the presence of
Haemoglobin
The presence of iron in haemoglobin provides a reddish colour to the blood (RBC). Oxygen binds to the haemoglobin (oxyhaemoglobin) and is transported to the different parts of the body. Similar to oxygen, even carbon dioxide is transported by the blood. This is also accomplished by carbon dioxide binding to haemoglobin
(carbaminohaemoglobin).