How strange Rob. The idea of thinking that red and green indicated some form of traffic light system never even entered my head. I just look at the buttons from the point of view as to what side of the point they are placed. To my thought processes, the colour was completely immaterial. They could even have been the same colour it doesn't matter to me.
However, I think Hornby originally chose Red and Green for the buttons because (if wired the correct way round), the colours represent Hornby's standard Red & Green point wiring colours. That is to say, clicking the Red button puts an operating pulse on the Red wire, and Green on Green.
Clicking the Red button, puts an output pulse of the R8247 + terminal. This is the terminal that is normally used for the Red wire**, well that is if the motor is mounted in the correct orientation. Of course if the motor is mounted 180° on the point then the wires are reversed.
Note** although there is no official Hornby documentation I can find that confirms this.