Refer to figure.
To ensure that each person operating an aircraft does so within the operating limitations prescribed for it, the FAA requires that there is available in it a current Flight Manual, appropriate instrument marking and placards, or any combination thereof.

The purpose of the flight manual, markings and placards is to detail, for the operator of the aircraft, the operational limitations prescribed for the aircraft.

In lieu of a flight manual most amateur builders prefer to mark their instruments and to affix the necessary placards to the instrument panel as the primary means for complying with these requirements.

Airspeed Instrument Markings

• The airspeed indicator should be marked with a radial red line to establish the never-exceed speed (Vne).
• The takeoff and any precautionary range should be marked with a yellow arc.
• The normal range is marked with a green arc.
• The flap actuation range is marked with a white arc.

The meaning of the red range on the airspeed indicator is Speed which must not be exceeded regardless of circumstances.