A novel technique is reported for fabricating silicon/polymer composite nanopost arrays by combining colloidal lithography and surface-initiated atom-transfer radical polymerization. The composite nanopost arrays possess a core/shell nanoarchitecture, with shells of poly(2-hydroxyethyl methacrylate) and cores of silicon nanoposts. The polymer brush possesses quasi-3D homogeneous nanoarchitectures due to the controllable polymerization process using the surface-initiated atom-transfer radical polymerization technique. The composite nanopost arrays are durable due to the particular nanoarchitectures. The backbone templates of the composites are silicon nanopost arrays directly etched from silicon substrates, and the polymer shell is covalently grafted from the arrays. The composite nanopost arrays exhibit vivid colors. Moreover, the colors of the composite nanopost arrays can be tuned from green to red by changing the thickness of film. Specifically, the composite nanopost arrays can be used as sensors to rapidly detect water vapors with high stability and reproducibility. Many different functional surfaces could be prepared through this technique using other functional monomers.