Answer:
Inverting a tube of blood will not prevent it clotting. Only an anticoagulant will do this, the different coloured tops on blood samples refer to the different anticoagulants or lack thereof inside the tubes. Inverting the tube is done to mix blood samples that have settled gently inverting the tube 6-10 times is enough to mix the blood. Inverting the tube more than this or doing it too violently could trigger clotting even in a sample containing an anticoagulant.