This National Marker Company sign with the legend âCAUTION - NON POTABLE WATER - DO NOT DRINKâ is often used near open or contaminated water sources to inform people that the water is not safe to drink. The caution sign shows the message in yellow text on a black base with black text on a yellow base. The bold and legible text and contrasting colors make the sign easily readable even from a distance. The caution signage is compliant with OSHA standard and has an NMC Sign Muscle coating that protects the text against fading, scratches, scuffing, mold, and moisture.