SP Single Color Light Signal System query

[Meiriongwril]

Can anyone explain how the SP Single Color Light signals are used? If you have only one lamp, how do you show red/orange/green? Did they always come with one lamp above a second? - and if so, were two colors used together to mark a particular state of the road?
Any help welcome on this!

 

[SouthernPacific]

If im not mistaken, SP target signals have one bulb, and the red, yellow and green lenses either flip in front, or rotate around the bulb.
Thom