Indicator light strips
The ones leading from a button to the tick/cross sign by the door.
The sign with the tick/cross can be a func_brush toggled with an env_texturetoggle, this is easy enough and I have it working.
The lights however it seems you must do with overlays, as weird things happen when you apply the texture to a brush. My problem is that info_overlay does not accept any inputs/outputs so you can't hook it up to an env_texturetoggle. So anyone have any idea how we can get the lights that change colour?
Looking at the .vmt it has a Toggletexture like the X/tick sign, just how do we trigger it?
Didn't ValvE use some kind of special entity for this? Dunno what it's called tho.
I'd like to be corrected on this, but I think it's yet another thing we can't access till the full sdk release.
I just looked at one of the official maps, the light strips are overlays and there is a special entity they use to access it and change it's texture.
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Wednesday I plan on writing a tutorial for overlays as well since they seem to be so widely used in Portal. Like I said I don't have work wednesday so I plan to utalize all that time for something useful. 
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That's if I don't beat you to it.
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Unless anyone else has had better luck, this may have to wait for the SDK
I took one of their maps apart to figure out how this is done. It appears to be an env_texturetoggle targeting a info_overlay_accessor, this looks like a regular overlay entity with a couple of extra properties. I assume we don't have access to this because it is in the updated base.fgd, which we should get with the new SDK.
Anyways the new properties are targetname (self explanatory) and OverlayID, which from the map I looked at just appears to be a unique id.
I copied the info_overlay from base.fgd to the bottom of portal.fgd and added these changes, then loaded up hammer and changed some of my overlays to the accessor version. This worked find in hammer, but so far I cannot get them working in game, they just don't show up.
What is really odd is some of the regular info_overlays in the map I didn't change to flip from blue to orange when I press the button, despite not being connected.... 