B uildings are important users of energy in present-day society, and energy-efficient buildings play an increasingly important role in sustainability 1. More than 76% of total electricity and 40% of all energy in the United States are consumed to provide comfortable, well-lit and well-conditioned buildings 2 , which results in a cost of more than $430 billion annually and substantial global greenhouse gas emission 3,4. The heating, ventilation and airconditioning (HVAC) systems of buildings contribute to about 40% of the building energy consumption, which is directly related to the heating and cooling demands of buildings 5. This huge energy consumption is causing severe environmental and economic problems. It is therefore essential to develop innovations of science and technology to achieve improved energy efficiency of building HVAC with a reduced carbon footprint. In addition to building insulation materials with low thermal conductivity such as expanded polystyrene, vacuum insulation panels and aerogels 6 , engineering the radiative properties of building envelopes is emerging as a promising approach for building energy savings. For instance, near-infrared (NIR) reflective cooling coatings have been developed. These coatings reflect the invisible NIR light in the solar spectrum to decrease solar heating, but they show visible colours 7. Additionally, passive daytime radia-tive cooling materials create a surface that has high solar reflec-tance and high emittance in the mid-infrared (MIR) transparent window of the atmosphere for saving cooling energy by rejecting solar heating and radiating heat to the cold sky 4,8-12. These materials are suitable for unshaded building roofs to efficiently radiate heat to the outer space or sky, and the ideal installation locations should have long and warm summers to avoid a heating penalty in winters 13,14. For building walls, unlike for roofs facing the sky, the MIR radiative heat exchange is more dominant with outdoor ambient surroundings than with the sky 15. Such radiative heat exchange is more pronounced in urban areas, where buildings are concentrated. Traditional building materials usually show high thermal emissiv-ity 16,17 , leading to intense radiative heat exchange. In hot climates, the overheating caused by high radiation throughput from the hot environment and the sun results in increased cooling demand. Similarly, the excessive radiative heat loss from the building interior in cold climates leads to increased heating energy consumption. A design that minimizes radiative heat transfer through the building wall envelope will therefore be beneficial for both cooling and heating energy savings throughout the year 18. Applying low-emissivity materials to the building envelope has been shown to be a promising strategy 19,20. Metalized and metal films or foils (laminated with other materials) 21,22 and low-emissivity paints containing heat-reflective aluminium (Al) or silver dust as radiant barriers have been proposed to be installed with sufficient air layers, usually within the building envelopes 23. These materials create more reflective surfaces for hollow bricks 24 , roof attics 25 and exterior or interior wall surfaces 26-28. However, regardless of their thermal performance, the state-of-the-art low-emissivity materials are commonly a metallized silver colour and suffer from severe aesthetical limitation, which tremendously hinders their extensive practical applications. It is important to develop materials that satisfy people's aesthetical demands while still achieving energy-efficient buildings. Here we develop a category of coloured low-emissivity films for building wall thermal envelopes, providing a solution to year-round building heating and cooling energy savings. These coloured low-emissivity films aim to realize the thermal regulation and Buildings are responsible for over 40% of total US energy use, of which about 40% is directly related to the operation of heating , ventilation and airconditioning (HVAC) systems. Saving energy to heat and cool buildings would contribute substantially to sustainability. Here we propose a category of coloured low-emissivity films for building walls that constitute the main component of the building thermal envelope. We demonstrate high reflectance (~90%) in infrared wavelength range and selective reflectance in the visible light wavelength range for desired colours. These films can help minimize radiative heat exchange between the indoor and outdoor environments, thus saving energy for all-year cooling and heating while satisfying the required aesthetical effect. Simulations show that these films can help reduce heat gain and loss by up to 257.6 MJ per installation wall area annually. In the case of a typical midrise apartment building, the HVAC system can save up to 124.46 GJ (equal to 9.87% of the building's HVAC energy consumption). By rough estimation, a global CO 2 emission reduction of 1.14 billion metric tons annually could be achieved. Our work provides insights for innovative energy-saving building envelope materials that can help achieve global carbon neutrality and sustainability.