I demonstrate how a combination of self-templating assembly and peptide amphiphile permits facile fabrication of biomimetic colorimetric sensors for aldehyde gas detection. Lysine-rich peptide amphiphiles (LRPAs) were designed to self-assemble into nanofibers. These nanofibers were deposited as nanofiber films on a solid substrate via controlled evaporation, resulting in distinctive structural color band formation. When exposed to aldehyde gases, the LRPA-based color band exhibited concentration-dependent color change, which can be attributed to the structural change of the film upon specific binding of aldehydes to the lysine motifs. Furthermore, from principal component analysis of the color change data, we confirmed that the LRPA-band patterns exhibit selectivity toward aldehydes over non-aldehyde gases. We believe that our strategy can be applied to the development of inexpensive, rapid, and user-friendly aldehyde detection systems with on-site monitoring capabilities.