Space cooling consumes significant amounts of energy and is a major driver of peak electricity demand in HK buildings. Passive radiative cooling requires no electricity input and operates without refrigerants, making it a promising solution for smart-green buildings. Generally, passive radiative cooling is the process of heat removal from a sky-facing surface to the universe, where the temperature is -270 ⁰C, through radiation, cooling the surface below ambient temperature. It is a natural phenomenon happening everywhere, inspiring many applications, transforming our lifestyles (e.g. frost forms on surfaces at night even though the ambient temperature remains above freezing point). This project aims to develop a low-cost high performance passive radiative cooling paint (PRCP) that can be practically used as a cooling source in buildings for lowering energy consumption of traditional air-conditioning systems and reducing environmental pollution. Our preliminary results show that the proposed PRCP can reduce the indoor air temperature of a model house by ~2⁰C, providing an energy-saving of ~10% of traditional air-conditioning systems. Notably, the overall cost is only ~HK$15 per m2, leading to have a payback period of less than a few years. Overall, the success of this project will inspire future architectural and engineering design to reach smart-green buildings.