purple, brown and yellow after giving blood?

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It is possible that you may have a small hematoma or a bruise (minor bleeding within tissues) at the site of venesection, done while you were giving blood.

Bruises go through changes in the color as the tissue begins to heal itself and the hemoglobin in the blood undergoes breakdown and absorption by the tissue phagocytes.

When blood first collects in the tissue, a bruise has a reddish appearance. Within 1 or 2 days, the hemoglobin in the blood changes and your bruise turns bluish-purple or even blackish. After 5 to 10 days, the bruise turns greenish or yellowish and turns yellowish-brown or light brown after 10 or 14 days.

The complete healing of the bruise occurs after about 2 weeks.