RBC is red in colour due to the presence of:

Hemoglobin (Hb or Hgb) is an iron-containing protein found in practically all vertebrates' erythrocytes or red blood cells (RBC). Each haemoglobin molecule is made up of four subunits: two alpha and two beta. Each subunit encircles the core heme group, which contains iton and bonds to one oxygen molecule. Because of the interaction of iron and oxygen, blood cells are red. As a result, the presence of haemoglobin is responsible for the red colour of red blood cells.