Question

In a coloured diagram why do we generally show pulmonary artery in blue and pulmonary vein in red

Answer 1

pulmonary artery takes deoxygenated blood away from heart to lungs and pulmonary vein takes oxygenated blood to heart

oxygenated blood is red
deoxygenated blood is blue

Answer 2

We know that veins carry deoxygenated blood whereas arteriescarry oxygenated blood.
But in case of pulmonary artery and veins its opposite, Therefore, pulmonary artery carrys deoxygenated blood from heart to lungs for oxygenation..
Thus unlike every other artery which is red in colour its blue in colour.