See Power behind the throne

"...éminence grise (English: "grey eminence"), a powerful advisor or decision-maker who operates secretly or otherwise unofficially. This phrase originally referred to Cardinal de Richelieu's right hand man, François Leclerc du Tremblay, a Capuchin priest who wore gray robes."

His biography in English is Grey Eminence, byAldous Huxley

For an illustration, see the painting "L'Éminence Grise" 1873 by Jean-Léon Gérôme at [1] or [2]

Note: That painting captures well the contrast between the colors of court and the dark robes of the monk, which may be "grey" even though they are brown. Capuchin monks today wear brown (perhaps they always did). Their brown color is often mentioned to explain the name of the coffee drink "cappuccino," which also suggests the monk's cappuccio, “hood,” and the cap of foam on the coffee. See http://www.billcasselman.com/wording_room/cappuccino.htm