Hazmat is a shortened term meaning "hazardous material." HAZ-MAT
Hazardous material can be anything that can cause harm to living things. Some types of hazardous material include biological, chemical or physical items. They can pose a danger to individual health and to the environment. These items, by the Occupational Safety and Health Administration (OSHA) can be agents that contain carcinogens (cancer-causing substances), toxic agents, corrosives, explosive or flammable chemicals and unstable agents.
The colours of each diamond in a way has reference to its hazard IE:
Flammable = red. (Although bags at nursing homes were red and not flammable).
Explosive = orange, because mixing red (flammable) with yellow (oxidising agent) creates orange.
Non Flammable Non Toxic Gas = green, due to all compressed air vessels being this colour in France after World War II.
France is where the diamond system of HazMat identification originated.