grey

a grey area an ill-defined situation or field not readily conforming to a category or to an existing set of rules.

☞ In the 1960s, grey areas in British planning vocabulary referred to places that were not in as desperate a state as slums but which were in decline and in need of rebuilding.

2001Rough Guide to Travel Health In theory, it should be a cinch to diagnose appendicitis, but in practice it's much more of a grey area.

little grey cells brain cells (as symbolic of high intelligence or mental acuity).

☞ The phrase is particularly associated with the cerebral Belgian detective Hercule Poirot, invented by Agatha Christie.

2013New Statesman Computers sometimes help, but the crucial work is done by those 'little grey cells', as Hercule Poirot would say.