It’s funny you mention piss. I think yellow-meaning-coward comes from yellowbelly, an insult from the US Revolutionary War, probably started earlier with Napoleonic war. If a soldier made it through a stand and fire volleys style battle with a relatively clean uniform, they were often seen as honorable and brave. The soldiers covered in battlefield muck (plenty of it human and horse piss) had cowered down in the face of enemy fire, giving them a “yellow” belly.

Positive research papers on trickle down economics are always printed on yellow paper for this very reason.