electricalswitch
The switch for the bathroom light is outside the room.
I want to put in a switch with a neon indicator that tells me that the light is on in the room so that users will switch it off.
There are only one blue, one brown and one earth wire at the switch.
Matching colors is usually correct, but there are exceptions. While this is considered the "right" way, it is for safety issues. The light will work either way. But we don't want the power from the brown going down the blue usually neutral in random parts of the wiring.
Which is why turning off the circuit with the switch is usually safe. If the wiring was reversed, with the switch on the blue neutral, the light will still work, but you would have gotten a really nasty full mains shock, instead of the "little" attenuated by dimmer shock. Always kill power to circuits at the breaker before working on them!
Dimmers, when full off, should indeed be full off. The fact it illuminates a light means it is defective. It needs to be replaced. It's not unusual for dimmers to emit a slight 50 hz hum during use, the volume proportional to the current being passed. But they must be able to completely open the circuit.
If you do get another dimmer instead of a regular switch, do ensure the lamps in the fixture are dimmable, and the dimmer is compatible with the lamp type.
You need one or two 3-way switches with pilot lights. These are also called 3-way switches, illuminated ON. Some have an illuminated toggle, some are decorative style and some have a separate light. These are examples (not that none of the ones shown below are actually 3 way switches)
These switches generally need a neutral wire at both switches. Conventional unlighted switches do not need a neutral. They also need three wires (other than neutral) running between the switches.
If you only want the pilot light in one location, you still need two 3-way switches, but only the lit one needs a neutral.
Best Answer
Your switch wiring seems to be a switch loop with no neutral at the switch location. In such situation it is possible to use a neon indicator switch that has a lighted indicator when the switch is OFF provided the light itself is an incandescent bulb. Other types of bulbs may not work correctly with this type of setup.
Your desired setup however is just the opposite. You want the neon indicator to be lighted when the switch is in the ON position. It will not be possible to safely wire up a neon indicator when there is just a switch loop pair at the switch location. In order to get it to work the way you want it will be necessary to run a replacement power cable from the light location to the switch that contains a neutral wire in addition to the two wires for the switch loop. When that replacement cable is in place the neon indicator is wired between the neutral wire and the switched wire that goes to the light fixture.
You would also want to double check that the other wire at the switch is the hot line. If it is not then the light fixture itself is wired up wrong and should be corrected. Proper switch loop wiring feeds the hot wire to the switch and the return back to the light and then the opposite side of the light to the neutral at the light location. This assures that when the light is off there are no live connections inside the light bulb socket.