... And I meant the southern colours in the interior
Southeastern is going blue now and C2C likewise AGA only went white as their framchises were due to change
The 387s when ordered were a short-term fix to get over the DfT's delays in ordering the class 700s. The 377/6 and 377/7s were the last Electrostars ordered for GTR services until the Gatwick Express 387/2s. Electrostars delivered from the 377/6 &7 batches are different in many ways from the 375s and 377/1 to 377/5s. When delivered, they weren't even software compatible with the earlier stock according to postings here at that time. The 379s and 387s were also of the newer design systems. So with a modular interior design, it is not surprising that they are similar to each other and rather different to the earlier Electrostars. With such a modular design, the fixed structures inside are left as standard, and TOCs will just play tunes on the seat cover colours.
As the 29 class 387s were destined for ultimate use on OLE lines, (GWML, MML or Kings Cross outer suburban post Thameslink Programme, they were ordered in 'vanilla' paint scheme and as far as I am aware, there are no Soutern EMUs or DMUs painted thus. The green on the doors is of no consequence as it can be changed much more easily than the whole gerrn ends of other Southern coaches. There is no equivalent for interiors so the 377/6 & 377/7 schemes which had just passed through the factory were used. Clearly this pleased GTR as they were proudly telling everybody on Thameslink routes that they had new trains (which looked similar to but better than the blue/pink exterior 377/5s that were loaned when FCC required more capacity).
Interiors get changed fairly quickly, usually as part of a branding exercise (I've travelled on several 319s since they were 'refreshed' for Northern and they do look sufficiently fresh and Northen'ish for passengers there to see them as their own). Just look at the myriad of liveries both outside and in, that the current Thameslink trains have:
319s in FCC blue/pink
319/2s in Southern greens
319s in white with blue doors
319s in white with pink doors
377/2s in Southern Green
377/5s in FCC blue/pink
387/1s in white with green doors
387/2s in Gatwick Express red
and from next month, 700s in white/grey, (until the marketeers paint them something else)
At the end of the day, colours don't make any real difference to passengers, - it's only the enthusiasts and the marketeers who really care.
In 18 months, the 29 Class387s will be plying GWML routes with whatever colurs GWR want to use to claim that they are 'brand' new.