Is that other machine located in the same network segment as the Raspberry Pi? If so, are its IP and subnet mask set properly?
In order to be able to properly debug your issue, please provide us with additional information on what your network setup looks like, especially what the IP assignments of the IPFire interface looks like, and how IP addresses are allocated to the network the “other machine” is located in (e. g., via DHCP, or statically assigned).
since the blue network is isolated, you either have to plug the computer into the blue network or assign the blue interface to the green network (temporarily for configuring) in ‘zone configuration’ of ipfire.