Dutch and Flemish masterpieces, French impressionists and Postimpressionists and American art from colonial times to the present are featured here.   It is among Washington's oldest art collections .Open: Wednesday-Monday 10 a.m.-5 p.m. (to 9 p.m. Thursday). Admission: Donation: adults $3, families $5. Free guided tours daily at 12:30 p.m. Group tours by appointment; call (202) 639-1730 Monday-Friday 10 a.m.-5 p.m  Metro: Farragut North, Farragut West.  Related Links: Corcoran Gallery of Art