The grey areas are for crews only - some are less desirable to be nearby (due to mechanical noises, vibrations, etc.), while others you wouldn't even notice. Usually you can guess what the grey areas are for. For example, the large grey areas on deck 3 & 4, between mid and aft, are most likely the galley. The small grey area immediately behind the fore & aft elevator banks on each deck are probably for storages & cleaning supplies and service elevators, and also to buffer the elevator noises from the adjacent cabins. The large grey areas on deck 1 and below should contain the engine, supplies, food storage, crew cabins, loading/unloading equipments, etc. It would be interesting and informative (for cabin selection purposes) if anyone has a official map on the functions of the grey areas.