Both hybrid and composite metal nanoparticles (NPs) are considered promising platforms for colorimetric sensing due to their notable sensitivity, stability and functionality. In this chapter, we focus on the hybridization of metal NPs, including gold (Au) and silver (Ag), with diverse organic and inorganic materials to design hybrid nanostructures that enhance detection performance. By exploiting the synergistic plasmonic effects of core–shell and multi-metallic architectures, this approach allows the precise and selective detection of biogenic materials, organic compounds, and hazardous elements. Hybrid systems including Au@Fe₃O₄ and Au@Pt nanoparticles, along with their potential applications in diagnostics, environmental monitoring, and biotechnology are examined in this chapter. The exploration of such systems may significantly impact multiplexed colorimetric assays, real-time detection, and high-throughput screening, all of which benefit from the unique properties of composite nanostructures. This chapter discusses recent advances, existing challenges, and future prospects in hybrid nanoparticle-based sensing, emphasizing their potential to revolutionize diagnostics, biosensing technologies, and environmental safety monitoring.