Switch lanterns- what determines lens colors?

There's no single answer for this.  There was wide variance in the details of switch lights on different railroads.  Generally, a green light indicated that a main track switch was lined for movement on the through route (which was usually -- but not always -- the straight route).  Yellow lights were generally used instead of green on yard tracks.

The green light surrounded by the circular yellow collar ring could have been used at a location where it was necessary to line a flop-over or split point derail as well as lining the switch, reminding the Trainmen to do both, and also reminding Engineers approaching the switch to also look to see of the derail was properly lined before proceeding.  There are untold variations in switch lights, because there were no industry-wide standards, each railroad using what its management thought was the best.

Some more lens color discussions.

http://www.railroadsignals.us/signals/lenses/

NJCJOE posted:

Some more lens color discussions.

http://www.railroadsignals.us/signals/lenses/

The colors are the most confusing!

Cobalt Blue to the eye - called "RSA Purple"

Blue/Turquoise to the eye = "RSA Green"

The thing is - these were flame lit - and that flame spectrum worked with the glass shade to create the final color - so sometimes you can't just look at the unlit lens and know what color it is.

So in that first example you linked to - the green target appears to have blue lens - but that is correct.

These switch targets came from downtown Lubbock, TX.  I have never known the exact switches they came from, but their purpose is standard:  the Red-Green target was on a mainline switch with a diverging route to a secondary track such as a passing siding. The Yellow-Green target came from a secondary line switch with a diverging route to another secondary track.

Red=mainline. Yellow=secondary 

I have a green/red switch stand I got for nothing when the Penn Central was changing over to reflective types, this was in 1972, the maintainer said they were going to  be scrapped.  I scrubbed the heck out of it and repainted the metal parts.   And since these had low voltage lighting in them with enormous bulbs,  I converted mine to AC to use a 25 watt bulb,  installed a chain and is hanging by a floor joist in the basement.  Really looks good...

Marty