Yellow bile is the humour of the choleric temperament, and in Galenic physiology it was a residue separated out while blood was being made in the liver, and collected in the gall bladder. Its qualities are hot and dry, which is what places it with fire among the elements, with summer among the seasons, and with young adulthood among the ages of life.
Of the four humours, yellow bile is the one that mapped most easily onto something a physician could actually see. It was visible in vomit and in the yellowing of the skin, and fevers were understood as its excess, which is why the choleric complexion was thought the one most liable to acute and violent illness rather than to long decline. That observational anchor gave the humour a plausibility black bile never had.
The therapeutic logic ran as it did for blood. An excess of a hot dry humour called for cooling and moistening: a diet of cold and wet foods, rest, and preparations chosen for the opposite qualities. Whole pharmacopoeias were organised on this principle, with each simple assigned a quality and a degree of intensity, so that a remedy could be selected to answer a complexion rather than a named disease. Culpeper's English herbal, written in the seventeenth century for readers who could not afford a physician, still assumes exactly this framework and expects its readers to know their own complexion.