This is a great post at The Smithsonian Magazine on decoding what those symbols pasted as graffiti by utility workers mean:

These “safety colors” –expanded to include red, orange, yellow, green, blue, purple, brown, grey, white, and black– have been formalized by the American Standards Institute (ANSI) as Safety Color Code Z535, which provides Munsell notation and Pantone color-matching information to help ensure consistency across mediums.

While the color system warns workers about certain types of hazard, there is a complementary language used to approximately mark the underground location of a conduit, cable, or pipe. According to the Guidelines for Operator Facility Field Delineation established by the Common Ground Alliance, spray-painted lines (in the appropriate color, of course) space between four-feet and fifty-feet apart should be used to mark the center of a single facility or, if multiple conduits are running in a single trench, over their outside edges with arrows pointing in the the direction the services are running with a perpendicular line connecting the edge marks to form an H (as seen in the photo at the top of this post). A diamond is used instead of the perpendicular line to indicate a duct system.

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(hat tip: @pbump)