Single-coloured yellow flags were emblems of the following states: Johor, Brunei
Yellow is the colour of royalty throughout whole Southeast Asia and symbolizes often the ruling power of the sultan.
Single-coloured yellow flags can have very different meanings that go far beyond the topic of state or national flags:
• Signal flag Q of the flag alphabet
• A plain yellow was called pest flag in the old days, today it is called the quarantine flag. Meaning: Caution, risk of infection! Avoid this place, do not enter! Ships at anchor hoisted this, perhaps most famous simple, yellow flag when a contagious, dangerous disease had broken out on board and ship and crew were isolated / quarantined. Interestingly enough, there is not known any abuse of this flag, neither by navies, privateers, nor by pirates. Nobody dared to do that because the opposing party would have done it too, and the whole protective measure would have become ineffective.
• The yellow beach flag on bathing beaches, e.g. in Europe and the USA, signals to bathers: Caution! Bathing at your own risk and responsibility. Children, old people, non-swimmers, people with less swimming experience stay outside, do not go into the water, wind and currents can be dangerous! (A red-yellow horizontal striped bicolor indicates: lifeguards present on the monitored beach section, a black and white diced flag means: water sports area - swimming prohibited! Possibly danger from water sports equipment or vehicles!)
• In the American Civil War, for both parties a simple yellow flag marked the way to the military or civil hospital (some of them had been partially used by the military). The hospital itself was marked with a yellow flag with the green letter "H" for hospital in the middle of the flag. The medical staff were expected to wear a green sash to identify them and to indicate their auxiliary activity by carrying a yellow flag.
• In the 17th century, an unadorned yellow flag was the so-called colonel-flag in many armies. After it, the other, subsequent companies orientated themselves. At that era, each regiment a special colour was assigned as identification-colour for its flags (for example yellow, red, green or blue).
– T.Wise/G.Rosignoli, Flaggen und Standarten 1618-1900, München 1978
– R.Brzezinski/R.Hook, Die Armee Gustav Adolfs-Infanterie und Kavallerie, Königswinter, 2006
– R.Chartrand, Colonial American Troops 1610-1774 (3), Oxford 2002
about the yellow hospital flag and red ambulance flag:
– R. Hodges Jr/ P.Dennis, American Civil War Railroad Tactics, Oxford 2009
– P. Katcher/R.Scollins, Flags of the American Civil War (2) Union, London 1993
– P. Katcher/M.Youens, The Army of Northern Virginia, London 1975
about the red and black flags of the anarcho-syndicalists and the national-syndicalists:
– J.M. Bueno, Uniformes Militares de la Guerra Civil Espanola en Color, Madrid 1971