The Occupational Safety and Health Administration (OSHA) does not assign particular colors to hard hats, and no international convention exists that assigns meaning to the color worn. Some employers, however, do use the color of workers' hard hats to convey meaning, though this is strictly internal to the company or group of contractors.
Most companies that enforce a color code follow an informal standard for their hard hats. Yellow hats are commonly worn by general laborers. Blue is common among electrical workers. Green is sometimes worn by new or probationary employees, and white is reserved for supervisors and visitors to the work site. Additionally, many employers require workers' hard hats be painted in bright, high-visibility shades, and some require the application of reflective tape to the sides or back of the hard hat for easier identification at night.
These color codes developed informally and are far from universal. Molded plastic headgear became common only in the 1950s. Before that, fiberglass or aluminum hard hats made color coding difficult, as most surface paints degrade under the conditions where hard hats are used. One tradition that grew up around color is the practice of keeping a pink hard hat on the job site. This is lent out to workers who report to work having forgotten to bring their personal hard hat from home.