Depending on the rank of the person heraldry could actually be gold, even on a disposable item such as a shield. On a painted item this would mean gold leaf, certainly embroidery could have used cloth-of-gold fibre (dullish, but with a metallic sheen) or true gold thread.
If you want to represent paint then it's important to consider what pigments (pigment, singular, in many cases!) would have been available at any given period, just as one would with era-appropriate dye colours. So nothing remotely at brilliant as Cadmium Yellow; assuming you're aiming for accuracy think duller rather than brighter and you won't go far wrong - for example, the poorest classes of knight could easily have had an ochre in place of a true yellow.
Einion
