Ok here it is...
> In Renaissance-era Amsterdam, the surgeons used the colored stripes to indicate that they were prepared to bleed their patients (red), set bones or pull teeth (white), or give a shave if nothing more urgent was needed (blue).
I'm not sure that can be right, because I think blue's only used in the USA. (At least, I only see it with blue in emoji; in England they're red and white. Amusing if it is though, and the only still-relevant colour has been lost here!)