After a year of testing designs on bunting, planes, tanks, shoulder patches and letterheads, NATO last week adopted an official flag. Color: navy blue and white. Officially, the flag is said to be "a four pointed star representing the compass that keeps us on the right roadthe path of peaceand a circle representing the unity that binds together the 14 countries." But NATO Secretary General Lord Ismay of Britain was much less high flown in approving the design. "It is," said he, "simple and inoffensive."