I don't remember the section but the frequency flag is an FCC requirement. I believe it's in the Part 95 rules. The colors are red for aircraft on 72 mhz, yellow for surface on 75 mhz, black for all on 50 mhz and black and another color for each channel on 53 mhz for all. I am not sure currently about 27 mhz. In the past the 27 mhz channels had a two color flag like, orange/blue, red/black, etc.

Rich
AD9V