Traffic Light Protocol

The Traffic Light Protocol is a system for classifying sensitive information created in the early 2000s by the UK Government's National Infrastructure Security Co-ordination Centre, in order to encourage greater sharing of sensitive information.
The fundamental concept is for the originator to signal how widely they want their information to be circulated beyond the immediate recipient. It is designed to improve the flow of information between individuals, organizations or communities in a controlled and trusted way. It is important that everyone who handles TLP-labeled communications understands and obeys the rules of the protocol. Only then can trust be established and the benefits of information sharing realized. The TLP is based on the concept of the originator labeling information with one of four colors to indicate what further dissemination, if any, can be undertaken by the recipient. The recipient must consult the originator if wider dissemination is required.
A number of current specifications for TLP exist.

From ISO/IEC, as part of the Standard on Information security management for inter-sector and inter-organizational communications.
From FIRST, which formed a Special Interest Group to draft a standardized set of definitions for the TLP colors and guidance on their usage. Version 1.0 of the standard was published by FIRST on August 31, 2016, before being replaced by Version 2.0 on August 5, 2022.

CISA officially adopted Version 2.0 of the FIRST standard on November 1, 2022.

Summary of TLP's four colours and their meanings

There are four colors : RED - personal for named recipients only

AMBER - limited distribution
GREEN - community wide
CLEAR - unlimited, up until FIRST TLP 2.0: WHITE - unlimited

In practice, one will indicate a document's classification with the acronym "TLP", followed by a colon and classification level, for example: "TLP:RED".