Numerous optical films have been developed to implement optoelectronics with advanced performance. In this study, we propose a color purifying optical nanothin film that improves the performance of optoelectronics by filtering the white light to have a spectrum composed of pure three primary colors of red, green, and blue. It was experimentally confirmed that a wider color gamut that covers 176.33% of the sRGB could be expressed when the suggested optical nanothin film was applied to a display system consisting of a white back light unit and color filters. Furthermore, a full width half-maximum value of 20 nm on average was observed in the three primary colors. When this film was applied to light recording optoelectronics, such as cameras, it acts as a multiband-pass filter that increases the sensitivity of the three primary colors. The principle of this optical nanothin film is based on multiple light resonance inside the film. A theoretical analysis and simulations were conducted to design the structure of the nanothin film and optical characteristics were verified by both experiment and simulation. Because it is fabricated by in situ thermal evaporation it provides advantages in terms of fabrication time and cost, and it also has potential to be fabricated with well-established deposition equipment such as a sputter apparatus. The results of this paper show that nanoscaled thin films sufficiently control the optical phenomenon with a simple structure, implementing advanced optoelectronics.