A red flag means one thing on one coast and something stricter on another, and purple appears in some countries and nowhere else. The variation is historical rather than accidental.
Systems grew out of local lifesaving clubs
Beach flags predate any international body. Volunteer lifesaving organisations developed their own signals in the early days of organised surf rescue, and each did so independently.
Those conventions hardened as clubs trained generations of lifeguards. By the time coordination was proposed, several incompatible systems were already established and understood locally.
Changing a system carries risk. A population that has learned one meaning will misread a new one, so authorities are cautious about switching even toward a standard.
An international standard exists but is optional
A standardised set of flags has been published defining red, yellow and green conditions along with a distinct flag for hazardous marine life. Adoption is voluntary and national.
Some countries adopted it fully, others adopted the colours but kept local meanings, and others ignored it. Coastal safety is usually a regional competence, so uptake fragments further within a country.
The practical result is that a traveller cannot assume the system carries across a border. Neighbouring countries on the same sea often differ.
Double red is the most confusing case
In several systems a single red flag means dangerous conditions and strong swimmers should take care. Two red flags means the water is closed and entering it is an offence.
Elsewhere a single red already means closed, with no double-flag level above it. A visitor from the first system reads the single red as a caution and enters the water.
Lifeguards report this as one of the most common misunderstandings among foreign visitors. The colour is recognised and the severity is not.
Flags describe conditions, not the whole beach
A flag applies to the patrolled zone and to the moment it was raised. Conditions a few hundred metres along an unpatrolled stretch may be entirely different.
Chequered black and white flags, where used, mark areas reserved for surfboards and craft rather than indicating danger. Swimmers reading them as a general warning avoid safe water.
Flags also come down when patrols end. No flag means no assessment has been made, which is a weaker statement than green.
Reading the board beats reading the colour
Most managed beaches post an explanatory board at every access point showing the local system. It is the only reliable way to know what a colour means on that coast.
Boards also carry the day's specific hazards, which flags cannot express. Jellyfish presence, water quality notices and rip locations appear there first.
Where no board exists, asking the lifeguard is faster than guessing. The question is routine and the answer is specific to the tide that day.