Janus nanoparticles bearing two different properties on a single particle are amenable to self-assembly into higher-order structures via their directional interaction. We show that gold/silica Janus nanoparticles self-assemble in solution into clusters resembling colloidal micelles upon addition of a hydrophobic thiol which provides them with a surface active amphiphilic character. As the nanoparticles spontaneously assemble, the color of the solution evolves due to the coupling of the surface plasmons. Time resolved spectrophotometry in the visible and near-infrared ranges coupled to simulations were used to probe the assembly process. A singular value decomposition analysis reveals the presence of dimers as transient species. The structure of the clusters was probed using small angle X-ray revealing that the Janus nanoparticles assemble into clusters containing a few particles.