Final answer:
A blue flag is used to indicate potable water lines by utility companies. This is part of a color-coding system that helps identify public utility lines.
Step-by-step explanation:
A utility company flagging its utility lines would use a blue flag for potable water lines.
This color-coding system helps in identifying the types of services provided by the public utilities and prevents damage to these lines during excavation and other construction activities.
Public utilities such as water and electrical service providers are considered natural monopolies because it is impractical to have multiple sets of infrastructure for each provider due to the high cost and complexity.