Venules carry deoxygenated blood. All blood in humans is bright red when oxygenated, but, a darker red when deoxygenated. Deoxygenated blood is darker due to the difference in shape of the red blood cell when oxygen binds to haemoglobin in the blood cell (oxygenated) versus does not bind to it (deoxygenated).
Arteries typically carry oxygenated blood, which is bright red in colour. The exception is the pulmonary artery which carries deoxygenated blood from the heart to the lungs.
