As a rare visitor, and even rarer poster, to the forum I found this thread interesting and nostalgic. I cannot vouch for the current state of the 317 fleet, now living as I do (perhaps ironically) not that far from Eastleigh but in NX days I did play a significant part in the allocation of 317s to the West Anglia route in particular.
The 317/6s were all given their white/grey/blue livery with red doors in Prism days whilst most 317/1s (or /3s) were still in NSE livery in 2003. Some had the modified livery with the green stripe over the NSE red having spent time on the LT&S route (now c2c) - they were in generally poor condition by 2004 and then had an early repaint into 'one' livery when it was realised that their heavy overhaul dates were overdue.
The 317/7 Stansted Express units were in a distinctive metallic blue livery and, as has been said, racked up more miles than the rest of the fleet due to the intensive diagrams. All units were, in WAGN days, maintained at Hornsey although when 'one' started operation the aim was to move the West Anglia fleet to Ilford as quickly as possible. When the fleet was divided between 'WA' and 'GN' the initial plan was for a mix of /3 and /6 units to go to both Great Northern and West Anglia operations but as GN had the advantage of 365s whilst /6s were the best general purpose units on offer for WA, I personally persuaded the fleet team to allocate all /6s to WA.
Come 2005 with plans for a new timetable and it was obvious that additional capacity was needed on Stansted Express trains so some of the /3s were converted into 'Stansted Lite' units to work paired with a 317/7, dedicated to the airport service. At the same time some of the /3s became standard class only for the Hertford East service which was altered from operating via Seven Sisters to run via Tottenham Hale and the Lea Valley. This meant that all trains running down the Lea Valley were formed of basically the same kind of train, i.e. all 317s albeit with different internal layouts, so timetabling for service reliability was much better.
The 317/7s were declared surplus to requirements by Abellio when they took over the Greater Anglia franchise - the units went to storage in Eastleigh. They were extracted from there in 2015 for London Overground, another project I was heavily involved in, and were reliveried for the West Anglia Inners. As such the only initial internal change was the sealing up of the toilets - a quick and easy modification as the units were only expected to be in LO service for a couple of years. But that's another story...
M