Web proxy on both RED and GREEN interface

Hello. I am not very experienced in network setup. Is it possible to configure the proxy for the red and green interface?

welcome to the community.

No it is not possible to use the proxy for the red interface also.
What do you want to proxy there? The traffic reached your system, yet. Filtering and redirecting can only done by firewalling through iptables.
The proxy in IPFire ( squid ) handles outgoing traffic.
On red you would need a reverse proxy.