Optical resonances of nanoparticle have been studied for a long time in various optical devices. However, the difficulties in fabrication of uniform nanoparticles hinders the applications. Herein, we formed Si nanoparticles having a uniform size via laser irradiation on an amorphous-Si thin film and found the optical resonances of red, green, and blue (RGB) colors originated from the Si nanoparticles. Two-dimensional scanning of 355-nm wavelength of nanosecond laser with a Gaussian spot beam created Si nanoparticles of 100~200 nm at laser fluences of 150~200 mJ/cm2 . We demonstrated the color resonances could be tuned to red, green, and blue adjusting the laser fluence and scan pitch. The size and distribution are characterized by scanning electron microscopy (SEM), which revealed the Si nanoparticles are ellipsoidal shape, embedded in the residual Si layer. The optical properties are measured by dark field microscopy and fiber coupled spectroscopy. The RGB samples show peak wavelengths of 628 nm, 570 nm, and 495 nm, respectively, which are attributed to the dipole resonance as predicted by the Mie theory.