Rich is correct. Many railroads used speed signaling, in which the various aspects displayed indicate the speed at which the signal may be passed, and this is always a speed that protects against derailment if the train takes the diverging route (through the turnout). This sometimes requires 3-unit (3-head) signals. This was used a lot by northeast railroads and by Northern Pacific.
However, there is also another type of signaling which is known as route signaling, often used on western railroads. This used single-unit or 2-unit signals. On a 2-unit signal, the upper unit aspect indicates what the train will do if it proceeds on the main route (not necessarily straight track). The lower unit aspect indicates what the train will do if it takes the diverging route (which might be straight track). There are exceptions, such as double yellow, which tells the train to pass the next signal not exceeding 30 (sometimes 40) MPH and be prepared to enter a diverging route at the prescribed speed for the turnout. This same indication can be given by flashing yellow, or flashing yellow over red, aspects.
My suggestion is to see what the rules of the railroad you like best say, and, if it is feasible to follow that signal practice, be governed accordingly. If you want a stand-alone signal instead of a signal system, there may be limits to the number of aspects that can be displayed, just like real railroading.
Once you know what aspect you want to display, Terry Christopher at Custom Signals can be helpful. He is friendly, and really understands model railroading signaing.
