Colloidal crystal capillary columns (C(4)s; see cover) produced by pressure-assisted colloidal-microsphere assembly in capillaries (PACMAC) display a high degree of optical uniformity in the visible and near-IR spectral ranges and are able to spectroscopically differentiate alkanes by diffraction. The Figure shows sections of 180 nm (blue) and 225 nm diameter (green) polystyrene microsphere C(4)s, and the 225 nm diameter polystyrene microsphere C-4 after infiltration with ethanol (yellow).