Using color code in drawing always use for determining diffrences between elements and plan parts. By using that, It is possible to check the map/plan/drawing visually.
In an effort to achieve consistancy across files, the GSA (Region 5 has developed color assignments for MEP systems. All project data should conform to these to the extent possible within the authoring application absolutly in BIM process.
BIM Technical Standards
According to GSA all MEP files, regardless of software authoring platform, must follow the color coding defined on this text.
Unknown Pipe is only to be used when it is absolutely necessary. Only when existing pipe must be shown, but it’s contents are unknown, should a pipe be classified as unknown.
Applying these color coding completely depend on which platform you are using or client will accept.
Reference: GSA