250.119 Exception 1: Power-limited Class 2 or Class 3 cables, power-limited fire alarm cables, or communications cables containing only circuits operating at less than 50 volts where connected to equipment not required to be grounded in accordance with 250.112(I) shall be permitted to use a conductor with green insulation or green with one or more yellow stripes for other than equipment grounding purposes
Low voltage thermostats have used green for the fan control wire forever. White is usually call for heat.
In old phone wiring, the green on the line 1 pair goes to the tip, and the red goes to the ring. The ring may be grounded, but not the tip.
