This chapter comprehensively covers recent developments in the field of metal nanoparticle (MNP)-based colorimetric sensing platforms, with a focus on their applications for the detection of metal ions, biomolecules, and toxins. Various MNPs, such as AuNPs, AgNPs, PtNPs, and PdNPs, exhibit remarkable optical properties that enable localized surface plasmon resonance (LSPR) for rapid and sensitive detection with minimal instrumentation requirements. These colorimetric sensors offer significant advantages, including low detection limits, high selectivity, and the ability to provide visual and real-time monitoring. This chapter also explores the incorporation of MNPs with other advanced materials such as quantum-dots and hybrid nanostructures to enhance the performance parameters of the sensors. Despite the substantial progress achieved, several challenges remain, particularly in the areas of reproducibility, cost, and environmental stability. Future prospects indicate that substantial advancements can be realized through the development of smart sensors, hybrid nanoparticles and, the integration of artificial intelligence, potentially transforming colorimetric biosensing for biomedical, environmental, and industrial applications.