It looks like there is a standarised system used for marking trial trenching. However, archaeology practice is apparently less standarised. I'm not sure if this stage of digging conforms to general contruction standards or specific archaeology standards set by whatever company is doing the digging. Anyway, for interest here's the fairly typical colour coding system:
Red – Electric power lines, conduit and cables.
Orange – Telecommunication, alarm or signal lines.
Yellow – Gas, oil, steam, petroleum, or other flammable material.
Green – Sewage and drain lines.
Blue – Drinking water.
Purple – Reclaimed water, irrigation, and slurry lines.
Pink– Temporary survey markings, unknown/unidentified facilities.
White – Proposed excavation limits or route.
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