This little fungus is a light buff colour when it first emerges and then as it dries from the outer edge inwards it turns darker and so you get the two-toned effect of light centre and dark edge. Eventually, of course, it will become completely brown; hence the brown mottlegill (Panaeolina foenisecci). That said, there are other brown mottlegills so you need to be a little cautious here!
An abundant species on short grass so is often found on lawns in small troops. It will also occur in parks, on cricket pitches and on path edges, it is quite widespread. A summer and autumn species and is not edible.