If I use a NAS for file sharing and use SMB do Mac users loose all Metadata capabilities ? Specifically color labeling and the use of Spotlight searching ?
As always, it depends.
Most NAS OS's use Samba for SMB sharing, which supports OSX metadata in recent versions (Samba 4.4 and later, IIRC). But, you most likeliy you have to manually edit the Samba config file as usually there is no setting for this specific option in the NAS OS GUI.
I use a Synology NAS and after augmenting the smb.conf file according to https://support.apple.com/en-us/HT207128 (ignore the Final Cut specificity) I gained OSX metadata support over SMB.
Also, Synology supports file indexing, which is not the same as Spotlight, but enables Spotlight-like searching within the contents of shared folders.