Yellow is known in Heraldry, it's what they call gold. Of course you wouldn't colour a mundane shield in gold, you paint it yellow and your parchment would have gold, as it still does to this day, ditto for silver and white.
Your friendly proto-herald who has apparently read too much on this topic.
You know what annoys me? I have a 2:2 in History (this may be partially attributed to extreme exam anxiety, most of my college exams were sat in the college's sick room). I graduated in 1994, I apparently know a lot more bits of history that are obscure to so many people, I still read history, and I know so many people with "better" degrees than mine who haven't read much in the topic in years and have forgotten most of it.
I really have to find a topic and write somethings about them. I'm tired of yelling at the TV and books that haven't a clue.