Jan Wysocki wrote:

AFAIK roofing felt is there to:
a. stop the slates or tiles lifting in strong winds.
b. stop all the filth accumulating in your loft space (several tons of
dust in the 1903 terrace loft space that I hoovered out a few years ago.)

Those aren't the main reasons. The primary purpose of sarking (felt or
boards) is to catch wind blown rain and fine snow that inevitably gets
under the roof covering sometimes and allow it to drain into the gutters.

The green material observed by the OP does indeed sound like a
high-quality vapour permeable 'felt' such as 'Tyvek'.

--
Andy