The only people unhappy with the shade of green that '30 was painted had unreliable memories or badly faded/colour-shifted photos as their evidence. A lot of effort was put into accurately matching a particular shade of green that the engine had carried shortly before withdrawal, based on paint samples recovered from areas of the cladding that were protected from exposure and damage due to weathering. We used to joke that the colour was called "wrong engine green" once people started making critical comments about it.
The only reason '01 was painted black was that the incorrect shade of green was supplied by mistake, and until the correct shade was available the advice from our paint suppliers was that black would be a very suitable undercoat/base coat for the green paint. I liked the black paint because it was so much easier to make the engine look clean and presentable compared to the green.
As for repainting '01 in it's original green livery, I reckon the same comments as made above about the 35 apply. The engine has had numerous modifications since it carried that livery, so to be accurate you'd have to undo those modifications. That would include replacing the flat headlight goggle glass with domed glass, putting the superheater damper back on, putting the continuous blow-down back on, altering the location and arrangement of the regulator and superheater drains, removing the driver's side anti-glare window, and a number of other changes. Since these changes were made for practical reasons, I don't think there'd be much support for undoing them.
And in any case I would worry too much about how 3526 is painted in the future, as the asset manager at Thirlmere has stated she wants to stuff and mount the engine in the near future - same with 4001.
Cheers,
Mark.