The creation of white and multicoloured 3D-printed objects with high color fidelity via powder sintering CB processes is currently limited by discolouration from thermal sensitizers used in the printing process. Here, we circumvent this problem by using switchable, photochromic tungsten oxide nanoparticles, which are colorless even at high concentrations. Upon ultraviolet illumination, the tungsten oxide nanoparticles can be reversibly activated, making them highly absorbing in the infrared. Their strong infrared absorption upon activation renders them efficient photothermal sensitizers that can act as fusing agents for polymer powders in sintering-based 3D printing. The WO3 nanoparticles show fast activation times, and when mixed with polyamide powders, they exhibit a heating-to-color-change ratio greatly exceeding other sensitizers in the literature. Upon mixing with colored inks, powders containing WO3 display identical coloration to a pristine powder. This demonstrates the potential of WO3, and photochromic nanoparticles in general as a new class of material for advanced manufacturing.