14 CFR § 29.1549

Each maximum and, if applicable, minimum safe operating limit must be marked with a red line;

Each normal operating range must be marked as a green or unmarked range;

Each takeoff and precautionary range must be marked with a yellow range or yellow line;

Each engine or rotor range that is restricted because of excessive vibration stresses must be marked with red ranges or red lines; and

Each OEI limit or approved operating range must be marked to be clearly differentiated from the markings of paragraphs (a) through (d) of this section except that no marking is normally required for the 30-second OEI limit.