Final answer:
Gray water drainage piping should be marked with purple color to ensure that they are easily distinguishable from other types of pipes, which is important for safety and maintenance.
Step-by-step explanation:
Internal and external gray water drainage piping should be identified with purple piping or markers. The color coding of pipes is essential for safety, maintenance, and repair. Different colors are standardized to represent different types of contents. Gray water, which is wastewater that does not contain sewage (such as the water from sinks, showers, and washing machines), is identified by the color purple to distinguish it from potable water piping and other types of waste systems.