Using Red/ Green bulbs instead of clear on PREWAR Electric Box cabs power units

Not sure about engines, but I have a c. 1929 set that came to me with vintage red Christmas tree type bulbs in the passenger cars.  They are still in the cars today, nearly 15 years after I acquired the set.  The color looks awesome when running.

I would suspect that it is a personal preference of the owner, or possibly just the availability of bulbs back in the day.  If a bulb burned out and the owner happened to have Christmas bulbs available, they used them up, rather than going out and buying a clear bulb.