The black is from the coats. Long, short, open, closed. But black, almost always black. The puffy parkas of the seriously cold. The long topcoats of the multitasking and self-important (a lot of those around here). The dark suit jackets of those impervious to the chill.
Put them all together — the Hill types striding across the Capitol plaza; the office-worker at lunch — and you have a ballet, a choreography, a study in contrasts.
D.C. gets color from its tourists. But it gets its subtlety and its heft and its monochromatic harmony from its denizens.