A common strategy to optimize whiteness in living organisms consists in using 3D random networks with dense and polydisperse scattering elements constituted by relatively low refractive index materials. Inspired by these natural architectures, a fast and scalable method to produce highly scattering porous polymer films via phase separation is developed. By varying the molecular weight of the polymer, the morphology of the porous films is modified, and therefore their scattering properties are tuned. The achieved transport mean free paths are in the micrometer range, improving the scattering strength of analogous low refractive index systems, e.g., standard white paper, by an order of magnitude. The produced porous films show a broadband reflectivity of ≈75% while only 4 µm thick. In addition, the films are flexible and can be readily index-matched with water (i.e., they become transparent when wet), allowing for various applications such as coatings with tunable transmittance and responsive paints.