I don't think this is true as a blanket statement. If you get to all green, the ICE will be spun to burn off excess charge, but no fuel is used.
This is from Ken at Japan, the part about the green bars. If I recall, he said 1 cc of fuel is used (and he is one of the guys that has races on 1cc of fuel!) You certainly wouldn't want to avoid green bars buy doing something even more fuel consuming. I have edited the aid to point that out. Thank you.