Colorimetric plasmon sensors for naked-eye detection of molecular recognition events have been proposed. Here, 3-layered Ag nanoparticle (NP) sheets on a Au substrate fabricated using the Langmuir-Schaefer method were utilized as the detection substrates. A drastic color change was observed following the binding of Au NPs via avidin-biotin interactions at less than 30% surface coverage. The color change was attributed not only to the localized surface plasmon resonance (LSPR) of the adsorbed Au NPs but also to the multiple light trapping effect derived from the stratified Au and Ag NPs, as predicted by a finite-difference time-domain (FDTD) simulation. This plasmonic multi-color has great potential in the development of simple and highly sensitive diagnostic systems.