NFPA 704

NFPA 704, or the National Fire Protection Agency, in section 704 of their National Fire Code specifies a system used to identify hazardous materials. This system is actually a color coded diamond.

The diamond shape has red on top with the number 4, yellow to the right with the number 3, blue to the left with the number 2, and a white colored diamond at the bottom with a "W" and a horizontal line through it. Each color represents its own diamond within a bigger overall diamond shape. The blue, red and yellow fields are for health, flammability, and reactivity respectively. They all use a number ranging from zero to four, a value of zero means that the material poses essentially no hazard. A rating of four indicates that you are at extreme danger if exposed to such hazardous materials. The fourth value (the white diamond) tends to be a variable changing in its meaning, and what letters or numbers are written there.

The four divisions
The four divisions are encoded in four colors where blue indicates level of health hazard, red indicates flammability, yellow indicates reactivity (chemical), and white contains special codes for unique hazards.

Related References

 * NFPA 704 Wikipedia

NFPA 704