When signal is high Q1, which is an NPN transistor, turns on LED1. When signal is low Q2, a PNP transistor, turns on LED 2. IF signal is left floating out in the ether (not high nor low) then neither transistor is on. Both LED's will be lit but slightly dimmer. Not known if you'd discern the difference between 20 mA bright and 15 mA bright, but at 12 volts with R2 being 470Ω, the red will burn at 20 mA, the green at 18 mA or both in series at 15 mA.The base of Q2 needs to be more negative than the emitter by at least 0.7 volts. The signal line can't go below zero volts. When the signal line is high Q1 is On correct? Q1 Emitter will be at about the forward voltage drop of LED 1 and the Q1 base will be well over saturation point. So now I make my signal line Low. That will turn
Then all things considered I would think about an interrupt light beam similar to what we see used on garage doors for safety. The problem I see with for example a sensor like a magnetic proximity sensor or hall effect sensors is they have limited distance detection. This way an off the shelf turn key solution can be used in conjunction with the lights liked to earlier. I would also select something which will function in the environment of the loading dock in question.
According to your statement that the emitter of Q2 (PNP) must be at a lower voltage than the emitter. It is lower because 12 volts is present (minus forward voltage of the LED) at the emitter via the LED. Some current will flow from the emitter to the base, which is what turns a PNP transistor ON, conducting between emitter and collector - to ground. Voltage at the emitter of Q2 should be around 8 volts positive.