14 CFR §27.1545

1. (a)Each airspeed indicator must be marked as specified in paragraph (b) of this section, with the marks located at the corresponding indicated airspeeds.
2. (b)The following markings must be made:
   1. (1)A red line—
      1. (i)For rotorcraft other than helicopters, at VNE.
      2. (ii)For helicopters, at VNE (power-on).
      3. (iii)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).
   2. (2)[Reserved]
   3. (3)For the caution range, a yellow range.
   4. (4)For the normal operating range, a green or unmarked range.