Railroads call the indication of a signal the ASPECT. In simplest terms:
Green means go, red means stop, yellow means watch out!
But can be much more complicated than that.
Just google "railroad signal aspects". More info than you ever wanted to know.
These are a couple of good ones:
http://www.lundsten.dk/us_signaling/signalbasics/http://broadway.pennsyrr.com/Rail/Signal/The mainline signals stay the same for most railroads. These would be single target and dual target (leaving siding to single track main).
Where it gets real interesting is in what are called interlocking limits. These are typically three target signals with the lamps all in a vertical row. Think of three lamps, colors of Red, Green, Yellow, lunar (white) and the possibility of flashing one or more. Also in congested areas, like Seattle thru the tunnel and south to Black River special aspects will be common.
Around here most signals (with the exception of interlockers) remain dark until a train activates them. So if a signal is normally dark and then turns on, that means there is "probably" a train around somewhere that activated it.
Lucky Dog