FOG MACHINE!
The leaking green gas is actually a fog machine with a couple of lights shining into the fog. A single-room fog machine can be purchased at a reasonable price ($35). The fog input requires fog fluid to be input every 6-8 hours if running continuously.
BEYOND THE WALL!
There is a set of permenant ducts set up to allow for the fog to be transmit between the pipes on the other side. A set of sensors is attached at the entry point of the fog, where the connection is made on the other side. When the sensor is triggered, the light at the other end of the permenant duct will turn on in order to color the fog. Another option would be to have multiple fog machines if the fog gets too thin towards the end of the ducts.
THE PIPES!
The interior smoothness of the pipes will be critical to pass the fog through them. They will be painted to look like real metal, but will be made from a robust PVC piping. The joint diameters will be the same throughout, however, they will have different lengths to be secured at different locations to match the pattern on the opposite wall.