2-aspect coloured signal, 2 wires + common
Semaphore signals can be operated by wire in tube, solenoids or even motor driven systems with control circuitry similar to point switching. Colour light signals do have more scope for different methods of control. As always there is no one single correct method, just different ways of achieving similar results. With signalling on real railways, much of the complex circuitry is for the interlocking of points and signals to ensure safety and to prevent the signalman from setting up conflicting routes. If you want to replicate this accurately, the same circuitry employed by Network Rail would be required for your layout. A somewhat daunting task. However, if you are prepared to accept compromises on not having a foolproof system then read on.
2-aspect coloured signal, 1 wire + common
Model signals can use either lamps or light emitting diodes. Resistors may be required for either type depending on their stated specification. Resistors are especially useful with lamps as it prevents them glowing unrealistically bright, overheating and melting the signal top.
Searchlight signal, red/green two pole LED. One wire