Any organization or business that installs or operates cabling, electrical systems and other types of equipment needs to be careful about how exactly it warns people about the presence of potential hazards. Fortunately, the requirements for signage are set out in the ISO 3864 standard, an internationally recognized system for how to generate warning signs, and the ISO 7010 standard, which outlines which symbols to use.
If you want to be sure that things like the cable markers you intend to install properly convey a message of warning, keep these three factors in mind. This is especially the case if you want to make your own files and have a professional sign or printing shop produce items based on them.
There are four colors that are used to explain warning levels with a minimum of symbols and verbiage, known respectively as Signal Yellow, Signal Red, Signal Blue and Signal Green. Yellow denotes warnings and is signified by RGB hex code #F9A800. Red signals prohibitions or the presence of fire equipment, and its hex code is #9B2423. Blue signals mandatory actions need to be taken in an area, and it's represented by hex code #005387. Green is indicative of safe conditions, and its hex code is #237F52.
White (#ECECE7) and black (#2B2B2C) are also included in the color coding symbol. Black is reserved for text items and symbols, and white is used as a background for text.
When in doubt, you'll usually label something with Signal Yellow. Bear in mind that you should typically want to provide more warning than necessary, so using yellow for something like an underground fiber optic cable isn't the worst thing, even if fiber optics aren't as dangerous as electrical lines.
The general warning symbol is an exclamation point inside a yellow triangle. If you need to be more specific, you'll use a unique symbol. For example, a transformer label would include a lightning bolt symbol for an electrical hazard. All symbols in the ISO standard strive to be as non-verbal as possible, so you'll see obvious ideas conveyed in the form of things like a flame above a line to indicate flammable materials or a snowflake to represent low temperatures.
It's a good idea to include contact information for things like cable markers that aren't on a site you control. A phone number and a company name should suffice.