There are 3 main types of cells in the blood.

    •  Red blood cells

    •  White blood cells

    •  Blood platelets.

Out of which, major portion is red blood cells and that is why blood appears red in color.

NOTE:- Red color in these cells comes due to presence of hemoglobin which is an Iron compound. Iron has the property of reflecting red light. 

When hemoglobin is carrying a lot of oxygen (like when just leaving the lungs), blood is bright red. When most of the oxygen has been released to the body, blood is dark red.