hard to explain well but I will try:
1. color the pulmonary artery blue - the blood with CO2 is coming back into the heart to switch it for oxygen
2. the pulmonary vein in red - the blood in this vein is now full of oxygen and goes out to the rest of the body
3. the alveoli will be purple because this is where the gas exchange takes place (swapping CO2 for O2)