Standard:
19A Signal heads shall be dark green or yellow.
Support:
19B Visibility of signal indications, particularly red indications, is usually more important than visibility of the signal heads themselves. Green signal heads offer greater contrast to the signal indications. Yellow signal heads have application where contrast with the background is considered more important than contrast with the signal indications.
N.Y. Comp. Codes R. & Regs. Tit. 17 § 4D.12