Yellow is the American Public Works Association recommended color code for pipes containing natural gas, oil, steam, petroleum and other gaseous material. It is the standard developed by the Utility Location and Coordination Council with the American National Standards Institute, used by most states for marking underground utilities.
Most states require notification and color-coded flagging of any excavation areas near utility runs. The color-coding requirements for other utilities include red for electrical, orange for communication, blue for potable water, green for sewers and drains, and purple for reclaimed water and irrigation. Typically, companies require 48-hour advance notification before digging to allow for proper surveying and marking of the proposed dig site.