The IPFire documentation is a good place to start looking.
https://www.ipfire.org/docs/configuration/firewall/rules/port-forwarding
There is also an example given for port forwarding to a server on the green network.
https://www.ipfire.org/docs/configuration/firewall/rules/port-forwarding/red_to_server_on_green
Just replace the green server IP with the orange server IP.
There is also a page about the DMZ which also covers about setting up a port forward rule to a server on the orange network.
https://www.ipfire.org/docs/configuration/firewall/rules/dmz-setup