is it possible to connect a external WLAN Access point via the green network,
which is working as a blue network ?
Reason, the access point is far away from the base station. In addition,
later I will need more then one access point for the blue network. Is this also
I still have a green network, which I would like to have separated from the blue network.
As you want to keep the green and blue networks separate then I think the answer is no. Any WAP that you connect to Green could be connected to blue via a firewall rule but it would also have access to your green network.
The base station you mention, does this mean your IPFire box.
Do you mean that you have an ethernet cable network running from your IPFire green connection but not from the blue connection.
Could explain your setup with a bit more detail. That would help me decide if there are possible alternatives that might help you achieve what you want.
VLAN for BLUE network.
You will need appropriate hardware to do it.
NIC with vlan support and managed switch.