I can not find any place that talks about the difference between these two cover notations. It feels like grey cover has no meaning, the % to hit chance does not adjust at all whether or not you sit in "grey" cover for the accuracy that it shows.
Add to this that yellow cover does work(at last in some cases) and you have strange disconnect, where standing behind a wall from one angle gives you 100% cover, but if you mirror the scenario suddenly the 100% cover becomes 0%. This seems too massive for it to have come up nowhere before but I see no record of it being discussed. And it seems to have existed even in the base version of the game. Makes me believe this is a core problem with the engine or what is the explanation here?
Edit: I tested many of the tiles in the average city map, and I can understand most of the cases where you get "grey" cover. But the two cases that I just can not fathom is the car blocks(the 1 tile long chest high walls often lining the side of the roads), which give yellow/green/yellow from one direction, but only grey/green/grey from the other. how are they not identical from both sides? And it makes them utterly useless from one end because all it takes is two steps to the left/right and suddenly the block of pure concrete is non existent.
The other case is Full walls, which seem to always(not counting corners or other cases where something else interacts with them) Give Yellow cover from one direction, but grey from the other. (which is what prompted this topic in the first place) I just really can't figure out the reasoning. Why would hiding behind a wall from the south side or the north side change its effectiveness to protect you?