Vivid colors are demonstrated in silicon nanowires with diameters ranging from 105 to 346 nm. The nanowires are vertically arranged in a square lattice with a pitch of 400 nm and are electromagnetically coupled to each other, resulting in frequency-dependent reflection spectra. Since the coupling is dependent on the refractive index of the medium surrounding the nanowires, the arrays can be used for sensing. A simple sensor is demonstrated by observing the change in the reflected color with changing refractive index of the surrounding medium. A refractive index resolution of 5 × 10(-5) is achieved by analyzing bright-field images captured with an optical microscope equipped with a charge coupled device camera.