ISO 22324
ISO 22324:2015, "Societal security — Emergency management — Guidelines for colour-coded alerts", is an international standard developed by ISO/TC 292 Security and resilience. This document provide guidelines for color codes to indicate severity of hazards in public warnings. There are many different systems are already in use, such as colour state of meteorological condition or former Homeland Security Advisory System, and the standard is intended to be an approach to eliminate confusion due to such differences. Scope and contents. ISO 22324 includes the following main clauses: Color codes for severity level. Three basic color codes for severity levels  are defined with meaning and proposed action, as in following table. If more than three levels of hazards are needed, the standard recommends use of colors on the spectrum between the red and green. The number of levels should be minimized to avoid confusion, not to exceed seven colors. Diversity of human limitation of ability to recognize color, including color blindness, should be paid attention when a color code system is brought into operation. The standard recommends to pay special care to provide supporting information, such as: Color codes for supplementary information. The standard gives additional color codes for supplementary  information, defined with meaning and usage limitation, as in following table. Black and purple can be used to indicate special cases of danger, i.e. red color code, thus supporting information such as text (recommended color shown in table above) is also recommended. An example is the sequence black–red–yellow–green, used in triage tag in most part of the world. Recommended Color Selection. The Annex B (informational) of the standard gives recommended selection of color for above-defined color codes. Each color code has saturated and unsaturated cases, both defined in Munsell, CMYK, and RGB systems. Related standards. ISO  22324 is part of a series of ISO standards and Technical Reports on Emergency management, including