Passive radiative-cooling coatings often suffer from winter overcooling and parasitic heat exchange in urban environments because their infrared emissivity is fixed. Here, we report a colorful electrochromic infrared emissivity regulator (CECIER) for seasonally adaptive building envelopes. The regulator integrates an all-solid-state ITO/WO₃/LiNbO₃/NiO/Au multilayer device with infrared-transparent, porous polyethylene films that provide customizable colors and diffuse solar reflection. Voltage-driven Li⁺ intercalation into WO₃ nanocrystals induces localized surface plasmon resonance, while multilayer nanophotonic design tailors the emissivity spectrum. The device enables continuous modulation of infrared emission within atmospheric windows while retaining high reflectance in the 5-8 μm non-atmospheric band, allowing reversible switching between radiative cooling and thermal insulation (Fig. 1). At ±2 V, the average emissivity modulation reaches 0.33 in the 3-4.5 μm band and 0.45 in the 7-11 μm band, with a peak modulation of 0.61 at 8.7 μm. The regulator retains emissivity modulation after more than 1000 electrochemical cycles and 1,200 bending cycles. Outdoor experiments demonstrate temperature differences of approximately 2℃ at night and up to 3.1℃ during the day between the cooling and insulation modes. EnergyPlus simulations across 11 representative cities indicate annual HVAC energy savings of up to 3.46 MJ m^-2. These results establish a thin, flexible, and aesthetically customizable platform for all-season building thermal management.