The accepted image of Washington D.C. is that of a city consisting mainly of white marble buildings and monuments. The White House, the Washington, Lincoln and Jefferson Memorials all draw heavily on Greek and Palladian architecture. But neighborhood architecture is as varied as the people who live in our nation's capital. R Street is lined with rowhouses from about 1900. Block by block, they are being lovingly restored. Some houses now sport new coats of paint in cheerful colors that might have driven the original turn-of-the-century residents to collapse on a horsehair divan with a case of the vapors. In a way, bright yellow, peach or turquoise signifies a new vitality that is spreading, street by street.