Turns out that "YIELD" signs also used to be yellow. Whaddya know?
Look closely at this next sign. This guy is the inventor of the "YIELD" sign and the sign he is holding is inscribed "This is the first YIELD sign in the world".
The sign in this clipping below could very well be literally the sign held by the gentleman above ("The first YIELD sign in the world").
Well, even the old red ones still look cool...