Can't be too difficult to do - GWR repainted all blue interior panels in 150s/158s etc to grey and it makes a big difference.
Not too impressed by LNR's 350 refurbishment - would look much better if the main moquette was applied to the entire seat (instead of the plain grey sections), and with blue panels repainted to green or grey, London Midland vinyls removed, and possibly with fewer of the new screens. West Midlands Trains did a much better job refurbishing the ex-Overground 172s in my opinion, as did SWR with their similar 450s.
There seems to be a slight lack of consistency with LNR's refurbishments - for example, the 230s have a white-based livery instead of silver, and with mint green interior poles instead of the dark green in the 350s/730s.