14 CFR § 27.1545

Each airspeed indicator must be marked as specified in paragraph (b) of this section, with the marks located at the corresponding indicated airspeeds.

The following markings must be made:

A red line—

For rotorcraft other than helicopters, at VNE.

For helicopters, at VNE (power-on).

For helicopters, at VNE (power-off). If VNE (power-off) is less than VNE (power-on) and both are simultaneously displayed, the red line at VNE (power-off) must be clearly distinguishable from the red line at VNE (power-on).

[Reserved]

For the caution range, a yellow range.

For the normal operating range, a green or unmarked range.