A staccato prose poem to New York by novelist Whitehead (''John Henry Days''), ''Colossus'' thrums with anxious excitement and excited anxiety, accommodating the noirish, the reportorial, and the epigrammatic. In one chapter, we prepare to go out for the night: ''Twilight is a mask factory.'' Another begins: ''Such a multitude of stenches means it must be summer.'' The best passages deserve comparison with E.B. White's ''Here Is New York;'' the worst are small marvels of cryptic nonsense. (True, Broadway runs diagonally across the street grid, but how does it leave avenues ''appalled''?) And the broader lines on urban life are equally true of Chicago and London and Dar es Salaam. Still, Whitehead's deft sketches of the peculiarities of cocktail parties and subway platforms offer tingles of fond feeling.