Actually, the most preferred seems to be Blue. Usually the Order will state which if there is a preference (ime, usually there IS a preference). Sometimes, the preference may come buried in a page of instructions in the doc pkg. If not stated, I usually default to blue. I think they like the fact that blue sticks out. I have missed that info a few times & used the 'wrong' ink color...and nobody seemed to care. Just don't use glitter purple and you'll be OK.