Märklin produced a broad selection of color light signals and provided spare light bulbs for them for many years. I like to run signal LED lights instead of incandescent for a few reasons (power consumption, heat, longevity, ability to control brightness using pulse width modulation, and 3V).
Here is how I converted some old Märklin signals to use LEDs.
First, I remove the base, and unscrew the safety cage.
and and separate the channel that holds the wires from the rest of the pole by levering it away using the holes in the mast...
... resulting is these parts.
Then I separate these parts, discarding the small contact strips.
The holes in the housing are just the right size for 3mm LEDs. The appropriate color LEDs can be pushed in from behind.
Since I control my signals using a RemoteSign ESP, and they typically switch the positive side, I connect one wire to be the common negative for the LEDs in the signal head. The negative side of each LED is the side with the small flat edge on the rim of the LED. I insert the LEDs so that these negative sides can all connect together. All the positive sides must get their own wire, each uninsulated from all others. I use Kapton insulation to keep them electrically isolated from each other.
I make sure to leave room for the screw that hold the signal head together. I solder an appropriate color wire to each light (here a two aspect signal):
I solder a brown onto all the negative sides.
I push the wires back into the mast and thread them out the side of the mast above the lower screw hole.
I then solder a 220 Ohm resistor to each of the positive wires. (200 Ohms is appropriate when supplying the signal with 3.3V and produces a brightness that is not too bright.)
I then test the signal by connecting +3.3V to the color wires and the negative to the brown wire.
Once I know all the aspects work, it can be installed on the layout!