This study aims to optimize angular placements of the LEDs with novel cone-shaped caps for achieving high optical efficiency in an ultra-thin, directly-lit RGB LED backlight unit (BLU) for large-sized LCD-TVs. This novel lens cap is used as a diffuser with the purpose to gain higher efficiency and provide satisfactory uniformity over a display panel. To this aim, the outer surface of the novel lens is coated with aluminum for mirroring effects to reflect most of the LED emitted light horizontally and then reflect the light at the BLU boundaries, finally to the output plane. Since the emitted white light from LEDs result from color-mixing of three individual RGB chips in a LED package, the addition of the LED cap however deteriorates the aforementioned expected color mixing. The optimal design on angular placements of LEDs presented in this study for satisfactory color-mixing and emission uniformity is achieved by necessary optics simulations via TracePro, followed by utilizing an intelligent numerical optimization technique, genetic algorithm (GA). The design parameters for GA optimization are different combinations of LED placement angles in a backlight module. Favorable color balance is shown achievable in terms of high low color difference resulted. Finally, experiments are conducted, which successfully validate the expected performance of color balance and emission uniformity for a novel cone-shaped LED lens with optimized angular placements in a large-area backlight module.