Medieval painters built up a tremendous range of technical resources for obtaining brilliance and permanence. …
One of the most important services required of [yellow pigments] was to imitate the appearance of gold. Another of their chief functions was to modify the qualities of greens and, to a less extend, of reds. Of all their uses, perhaps the least important was to represent yellow things.
— The Materials and Techniques of Medieval Painting by Daniel V. Thompson (Page 174)
