Coloration agent for carbonyl sulfide, detecting means and fuel cell system

A coloration agent for carbonyl sulfide which comprises a metal oxide and, carried thereon, at least one metal component selected from among a cerium component, a silver component, a copper component, a nickel component and an iron component, or comprises at least one metal oxide selected from among a cerium oxide, a silver oxide, a copper oxide, a nickel oxide and an iron oxide; a detecting means for carbonyl sulfide using said coloration agent; and a fuel cell system equipped with said detecting means. The above coloration agent is capable of being colored by carbonyl sulfide, exhibits excellent capability performance as a coloration agent for carbonyl sulfide in a hydrocarbon fuel, and thus allows the visual judgment of the life of a desulfurizing agent.