Employers are not required to label pipes or piping systems under OSHA 29 CFR 1910.1200. However, other OSHA regulations and other federal, state, or local laws, regulations, or codes may require pipes and piping systems to be labeled or color coded.
If the employer does not have requirements, it may wish to consider the recommendations in American Society of Mechanical Engineers (ASME) A13.1, Scheme for the Identification of Piping Systems.