[AngronIsAngry]

Once you are at location and able to see ships and "lock on" (L-Key) your periscope onto those ships, a button on your command bar lights up. As for the color, that possibly depends on mods of the user interface. Maybe that's why you don't see a red icon.

Also, if you use automated targeting instead of manual; it doesn't suffice to just point the periscope at the target. The lock on has to be enabled.

You can use either periscope for making pictures (as the photograph icon is on their command bars too) and possibly the TBT on the bridge, but since most harbors have a couple warships or close by land based guns, I advise against using the TBT for pictures.

edit add:
The external camera isn't meant for photo recon. It really is an addition to your optics (periscopes and TBT) which is already build in at every sub at sub-delivery.

Hope that Helps