Ok, wow. Everyone is way off here.
All the lights on a single vertical mast are part of the same signal. Each light is _not_ independent of each other. The signal indications are based on the specific combinations of red/yellow/green lights.
In the case of the photo with the two signals at the end of the siding, the dwarf signal shows green on bottom, so that's a Slow to Clear signal (slow speed through the switch, then may proceed at track speed to next signal.) Dwarf signals generally allow slower speeds than the same combination of lights on a full mast signal.
On the mast at right, because it's a straight route, and there is no possible alternate route, there are only three possibilities: Clear, Approach (also called "Clear to Stop" in some rulebooks) and Stop. An absolute stop signal is Red over Red. Clear and Approach on this signal would be Green over Red and Yellow over Red respectively. So the bottom light is required to display the Absolute Stop (vs. "Stop and Proceed" that is possible on a simple block signal) and does not ever need to be anything other than red.
The same goes exactly for the signal protecting the diamond in another post. The bottom light can only ever be red since there is only one possible route through the interlocking. So the possible indications are Absolute Stop, Approach and Clear.
(Note that some railroads do have other variations on signalling, like Santa Fe's route signalling, Pennsy's position lights and B&O (or C&O?) Colour-Position Light signals)
