I never go on the London Overground even though I only live a few miles away from it (it's not that I don't like it. it just doesn't go anywhere I want to go) but I know quite a lot about it.
Transport for London bought it off silverlink and branded it part of the London Underground network (but it isn't underground so they called it the overground) so they have the same livery to london underground, but because their fleet of 313s is due for replacement any day soon (by 378s), I expect they cannot be bothered to liverise their whole fleet when it's just about to be replaced.
About the signs, LU have a different colour or colours for each line, and orange and white are the colours of the LO. I expect the 'Temporary Sign' is something to do with the massive upgrade that the LO network is going under. They're redoing all the stations and building an extention out to the east (through Brunel's tunnel under the Thames!).
It does seem very odd that some parts of the LO are 3rd rail while others are overhead, but actually, the line was electrified along with some parts of the Southern network, which is all 3rd rail DC. When the northern network was electrified, some sections were converted to overhead AC. but they never got round to converting the whole line. I don't know exactly the places where it changes, but camden road must be one of them.
You might also like to know why the 313s have such a low headroom and sunken pantograph trough (if you look in the middle carriage of each unit you will see the lower cieling at one end). this is because they are also used for inner-suburban services on the hertford loop and southern ECML (where I use them
) there is a branch off at Finsbury park to moorgate down a former branch of the Northern line (LU) the tunnel has a very low headroom, hence the need for 3rd rail power transmission and a low cieling.