We report in this paper the recent advances we obtained in optimizing a color image sensor based on the LSP technique. A device structure based on a a-SiC:H/ a-Si:H pin/pin tandem structure has been tested for a proper color separation process that takes advantage on the different filtering properties due to the different light penetration depth at different wavelengths inside the a-Si:H and a-SiC:H absorbers. Under reverse bias the green and the red images give, in comparison with previous tested structures, a weak response, while this structure shows a very good recognition of blue color, leaving a good margin for future device optimization in order to achieve a complete and satisfactory RGB image mapping. The physics behind the device functioning is explained by recurring to a numerical simulation of the internal electrical configuration of the device in dark and under different wavelength irradiations. Considerations about conduction band offsets, electrical field profiles and inversion layers will be taken into account to explain the optical and voltage bias dependence of collected photocurrent.