That sounds very weird and not normal.
What happens if you run the ping command for the green client from the IPFire command line.
Are the green clients getting dynamic IP addresses from IPFire, or have you defined Fixed Leases for them in the IPFire dhcp page or have you defined static IP’s on the clients themselves.
I am presuming that this means that after creating the firewall rule for the blue to green pinhole that you did press the Apply Changes button at the top of the Firewall Rules WUI page. (Clutching at basic straws as running out of ideas why your IPFire is performing so non standard.)