Here is one doctor's explanation found online: 1. Hemoglobin is responsible for transporting oxygen 2. For swimmers, the higher the hemoglobin concentration, the better the performance will be 3. A kind of enhancement drug will massively boosts hemoglobin concentration in blood 4. High concentrations of hemoglobin are reduced to high concentrations of deoxyhemoglobin after used 5. Before oxygen depletion, High concentration of hemoglobin will bring pink or red color to the skin 6. After oxygen depletion, High concentration of deoxyhemoglobin will bring purple color to the skin