It's a joke sign to make people not want to open your door.

I printed this without any color changing systems, I just added a pause by right clicking the layer line bar at layer 22 and manually taking out the filament and putting a different color in

You might not be able to scale down model at all with a 0.4 MM nozzle because the paint brush will disappear.

I attached it to the door with double sided Tape.