We demonstrate flexible transmissive structural color filters with enhanced color purity and brightness by exploiting resonance overlap within multiple cavities in penta-layered structures. Using an inverse design method that combines optimization and exhaustive search algorithms, the material choices and layer thicknesses are determined through a loss function based on the CIE XYZ color space, optimizing both color purity and brightness. The resulting color gamut is comparable to standard RGB, as assessed in the CIE xy color space, with luminance (Y from CIE 1931 model) values for the fabricated transmissive RGB colors being 0.14, 0.51, and 0.14. The contribution of each cavity is thoroughly analyzed using optical admittance diagrams and resonant mode calculations. Furthermore, the transmissive colors on a flexible substrate exhibit excellent durability, retaining consistent transmission efficiency and color purity even after 4,000 bending cycles and performing reliably at a bending radius of 5 mm. The versatility of this design approach makes them suitable for a wide range of applications, including e-paper displays, image sensors, flexible wavelength-selective optoelectronic devices, and decorations.