The battery SOC display on the Prius is only showing you the portion of the battery's charge capacity that the Prius will actually use, which is about 40% of what the battery is really capable of. The Prius will never deplete the battery completely (not even close), nor will it let the battery get fully charged (somewhat close).
However, the HV ECU will try to keep the battery SOC (State of Charge) at 6 bars when cruising. If you are going downhill and using brakes (or simulated drag), then MG2 (Motor/generator) will of course be used as a generator to slow you down, and that energy of course will be stored in the battery. At least until you have reached the max SOC that the ECU will allow, which is 8 bars, which again is not the true max of the battery. Any charge over that, and the system will try to 'burn off' the top charge by starting and stoping the ICE, which has MG1 use battery power to turn ICE to start. Bars 7 and 8 will show up as green.
Now if you are standing still then battery power is needed to operate the accessories. If you are going up a steep hill or rapidly accelerating for a prolonged time, then battery power is needed to provide the extra power/energy. Eventually the battery will hit usable bottom. But not to worry, the ECU will start ICE in the former situation, and renegotiate its strategy, mainly increasing ICE RPM in the latter situation. When the SOC is down to 2 or 1 bar, it will be red (purple, magenta, pink, whatever you want to say it is). Under no circumstances will the Prius allow the SOC to go down below sub 0 bars if it can help it. The only time it can't really help it is if you leave the car in READY while in Neutral for an extended time. In this situation, the inverter is not allowed to operate MG1/2, so ICE can't be started or if it is already running, its power can't be converted to electricity to recharge the battery. If you do happen to make this mistake, the ECU should shut the car down before the SOC gets so low that it no longer has enough power to start the ICE later.
So, is green bad? Well, it can be if you force it to conditions to require it to store energy when there were more efficient options. Remember, the conversion of energy from one form to another wastes some of that energy, so the less conversion the better. However, recouping energy that otherwise you couldn't avoid wasting, such as trying to slow down (alternative being not stoping/slowing down thus creating a hazard or accident), or ICE running to generate heat for the cabin, would be a good thing.
Similarly to running the battery down to 1 bar where ICE has to start. Doing this on purpose without being necessary is like running a regular car in idle for no purpose. But if you need the AC and you can't move (like I just dealt with being stuck behind an accident blocking the entire 3 lanes on a US highway) then what can you do? Just be reassured that since the AC compressor is electric, it will run at the slowest speed possible and still maintain the comfort you desire, and only run the ICE as necessary to maintain the minimum charge.
