Railroad trackside signal lights - Trains Magazine

We need a bit more detail here.  The 'incandescent bulb' (and then LED) signals with three colored aspects, like for example the UP Darth Vader heads, indeed have three separate bulbs or diode arrays.

A typical semaphore spectacle has three colored lenses, with a bulb and reflector arrangement behind them.  Ideally this hangs by gravity if motor power goes out, and the red light is arranged to be on with the blade as far down as it goes... what happens if the motor power also keeps the light lit is less often mentioned.

The signals like searchlight types have an interesting arrangement.  These have very powerful condensing optics inside that focus the light very tightly through the area where the colored lenses are -- these are very small, and much thicker than colored glass lenses in ordinary practice.  Red, for example, can almost look black when you're holding the thing.  Operation of these is as Dude said, in a motorized frame, with red being the intermediate position during a change.

Of course, one consequence of PTC is that any signals that rely on a motorized or active system to choose aspects are deprecated or forbidden, which is why the searchlights are going en masse and I believe the B&O style CPLs are coming up for progressive replacement... may it take decades more!  Same is true of any signal that can lose all aspects should a single bulb or element burn out... but less from a PTC than a PSR perspective.