Negative Indicator Lights
This was never done in Valve's Portal 2 maps, so I'm wondering if there's any official rule for something like this. Let's say that, for example, I have a floor button that toggles a tractor beam. But rather than the OnPressed output activating the beam, the beam deactivates when the button is pressed. The beam reactivates when the button is released.
What would be the appropriate indicator light setup for a scenario such as this? Should the lights and indicator panel be orange to start off with, but turn blue when the button is pressed? Or is it best to use the standard button layout in which the lights remain blue until the button is pressed?
I think both options are good.
Personally I?d go for start orange, switch to blue on pressed 
All right, thank you for the second opinion.
So would the tractor beam be blocking an exit? Or maybe be use to fling an object?
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The tractor beam scenario was just an arbitrary example. But in the context of the hypothetical situation, I suppose that's how it could be implemented. What I was actually doing was a test chamber section without a portal gun. A button, when pressed, opens one portal and closes the other. The primary reason why I chose to use an excursion funnel for the example was to make sure that an indicator panel's color would correspond to its indicator lights.
Players have been trained that blue = off and orange = on
I don't think you should seek to undo this training.
"Such monstrous sucking-constructions aren't built within one afternoon"
I don't think you should seek to undo this training.
Unless it is a faithplate 'cause then it is blue = on, orange is off 
haha yup that always annoyed me.
I think in terms of boolean variables. Generally, it makes things easier.
What would be the appropriate indicator light setup for a scenario such as this? Should the lights and indicator panel be orange to start off with, but turn blue when the button is pressed? Or is it best to use the standard button layout in which the lights remain blue until the button is pressed?
Well, it's not so difficult, if I have understood you well. I guess it should be something like:
1. Tractor beam must start enabled = YES
2. Prop_button setup:
OnPressed / [Tractor target name] / Disable
OnButtonReset / [Tractor target name] / Enable
OnButtonReset / [Tractor target name] / SetLinearForce = 250 (it's just an example if you want the blue funnel)
3. For the indicator lights, if you also want them to change the colour accordingly, you should only invert usual outputs on the prop_button for the Env_texturetoggle (if you normally use this entity for switching the indicator lights):
OnPressed / [Env_texturetoggle name] / SetTextureIndex = 0 (blue colour)
OnButton Reset / [Env_texturetoggle name] / SetTextureIndex = 1 (orange colour)
4. AND you may also need a Logic_auto for setting the "defaut" colour of the indicator lights to orange, like so:
OnMapSpawn / [Env_texturetoggle name] / SetTextureIndex = 1 (orange colour)
I'm not so sure that you'll need this 4th point as I haven't tried... maybe you'll have enough with the input of the prop_button that sets the colour to orange on UnPressed
I guess this should do... try and let me know
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[/spoiler]umm i dont think the OP was asking how to do it but rather wether or not they should.
I think in terms of boolean variables. Generally, it makes things easier.