14 CFR § 25.1549

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

Each normal operating range must be marked with a green arc or green line, not extending beyond the maximum and minimum safe limits;

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

Each engine, auxiliary power unit, or propeller speed range that is restricted because of excessive vibration stresses must be marked with red arcs or red lines.