Without physically checking in person can you tell from a photograph if a E86/E85 has the upgraded Nappa leather seats or will that need to be established from a Vin decoder listing the full specs and options fitted?
I think I'd only rely on a VIN decoder, although some colours were only offered in certain qualities.
My first two Z4s had Pearl Grey which was always the "basic" Oregon leather. Champagne was a BMW Individual Option in Nappa as were Syrah Blue and Amarone, but some shades can be misleading in photos.
Bi-colour seats were always "New England" leather which seems to have been somewhere between Oregon and Nappa.