Hugh Gourley, director of the Colby Museum of Art, passed from one room to another in the storage area of the museum's lower level and with a sweep of his hand noted the hundreds of works housed there. "It's safe to say some of these paintings have never been on view in our museum," he said. "We just haven't had the space to exhibit them."     A year from now, many of the paintings in that storage room will be on display in a new wing specifically designed to accommodate the museum's burgeoning collection. The product of a $1-million challenge grant from an anonymous donor, the 8,000-square-foot wing is part of a $3-million project that also includes endowment funds for security, conservation, traveling exhibitions and student assistantships. Construction on the $1.3-million building is scheduled to begin in June and be completed by next summer.      The wing will house six galleries on two levels and include roughly 150 of the museum's heretofore unseen gems. "This will allow us to show art from the mid-eighteenth century to the present, which we've never been able to do," Gourley said. "There will be a gallery for eighteenth-century portraits, another for nineteenth-century paintings, one for primitive American art, as well as rooms for American impressionists and early twentieth-century artists."     An expanded exhibition from The John Marin Collection also will be located in the wing, accommodating about two dozen etchings, paintings and photographs added to the collection in recent years, Gourley says. "We have the largest Marin collection of any academic museum in the country. The new gallery is three times as large as the current one [where the Marin works are exhibited]."     Los Angeles architect Frederick Fisher designed the new wing, opting for a simple Georgian structure. "Most of the collection is early American and we wanted a building that was consistent with that," said Fisher. "We imagined it as a house from that time period because most of this art was created for domestic environments."     Gourley says patrons will be pleased to finally see some of the museum's finest pieces on display. "Paintings are meant to be seen," he said. "Our collection will be much more visible now, not only here but in traveling exhibitions as well."