Reverse w/o microemulsions consisting of lauryl acrylate (LA), trimethylolpropane triacrylate (TMPTA), nonionic surfactant and aqueous salt solutions were used for the in situ generation of BiOX (X = Cl, I) nanoparticles and their integration into transparent nanocomposite materials via in situ polymerization of the monomer microemulsions. After particle formation, a photo initiator was added to the liquid mixture and radical photopolymerization of the oil phase was initiated by UV irradiation. The monomer microemulsions were chosen according to the requirements of the polymerization conditions and the stability of the microemulsions. Microemulsions were analyzed using temperature dependent DLS experiments; the nanoparticles were characterized using XRD measurements. After polymerization solid bulk polymer nanocomposite materials were obtained. UV/vis spectroscopy confirms excellent transparency of the composite materials compared to the pure PLA/PTMPTA samples. BiOI/PLA nanocomposite materials show a new photochromic effect, changing the color from yellow to black under UV irradiation and back to yellow after air exposure. The dynamics of the photochromic effect were investigated. The source of the photochromic effect was found to be connected with redox processes between the BiOI nanoparticles and the radical initiator in the material. XRD patterns of the reduced and oxidized particles were recorded. The photochromic effect was used to create temporary patterning in the colored nanocomposite material.