I am not very keen on the dark green chosen by GWR. That said, the painted ones look much better. The ones with vinyl look very dull - getting on for army tank colour dull.
You're right; in some shots the vinyled units can look almost black.
That said, I personally love GWR's branding idea; by going with a classy, dark colour they are really echoing the heritage of the GWR name. I certainly prefer it to when FirstGroup just slapped the word First in front of the name of the franchise and called it a day. I liked Dynamic Lines on the TPE Desiros but it really doesn't suit an HST in my opinion.
It's also far preferable to the current trend of primarily white liveries. Think Avanti, LNER, Northern, South Western, Thameslink, Great Northern, Southern, Southeastern, Greater Anglia, Transport for Wales and Greater Anglia (although I do like their livery on the FLIRTs).