Can you imagine an Ork with blue or yellow blood? The former would be invincible and the latter would either be incredibly rich or prone to self-detonation.
I have an idea for you. How about all you Orcs (yes, I did that again), hack each other to bits to see what colour your blood is?
Yes, dats a good idea, an you are a git. Also, remembah dat GREEN is da best colour, so's we is green through an through.