Thin films were prepared based on cellulose polymer doped with different ratios of natural dye derived from Portulaca grandiflora concentrations. The polymer and natural dye were extracted from eco-friendly materials—the cell walls of millet husks and Portulaca grandiflora, respectively. The spray pyrolysis technique was applied to prepare thin film samples to control the film morphology and reduce the roughness of the surface. Optical microscope and Fourier transform infrared were used to analyse structural, morphological, and functional groups for all samples, respectively. The peak absorbance, extinction coefficient, optical bandgap, Urbach energy, and optical conductivity for the thin films were determined using ultraviolet-visible spectroscopy. The results show an enhancement in the optical characteristics when the natural cellulose is doped with a dye. Doping cellulose with 5% P. grandiflora has led to a considerable reduction in the energy bandgap (to 1.95 eV), compared to the sample doped with 1%.