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Your boat's "flag" or "ensign" indicates which country your boat belongs to. Outside UK, that's not enough. Other countries insist on a certificate of registration to prove nationality, and they expect UK boats to fly a red flag with a union jack in one corner.
UK permits some boats and crews to fly blue or white ensigns. These symbols of status puzzle foreign officials; quite reasonably, since other nations use blue ones. And war ships do white. It is possible, when foreign, to use red, rather than white or blue. Saves questions about VAT . . .
It was UK practice to follow all sorts of flag etiquette, including lowering and raising flags at sunset and sunrise, sometimes accompanied by ceremonies, which may or may not include music, uniforms or drinking a tot. Ignoring this practice is legal, but shows you are not "people like us". This may or may not be the signal you wish to send.
Some countries require their national ensigns always to be flown.
Some countries will fine visitors who display very tatty courtesy flags.
All of the above behaviours and laws are regularly supported or rubbished, or diligently practiced or ignored. None of them will affect the performance of your boat. The hot air generated by proponents is sufficient to keep some flags flying.
Red canvas trousers, club ties or blazers are all legal. It's not clear whether these are causative, or just correlated with, flag etiquette opinions.