"The Perfect Tree,'' by MARIELUISE HUTCHINSON, in the "Small Works'' show at the Cahoon Museum of American Art, in Cotuit, Mass. (on Cape Cod) through Dec. 22. Green and white work well together. But usually it is brown and gray on the Cape in the winter, as the salt air and relatively mild temperatures assure that the ground is usually bare there in the winter, but with near-constant wind because of the presence or nearness of storms coming up from the southwest. Rather grim, except for the light traffic compared to the summer.