We demonstrated that a new intermediate composition of Ba1.55Ca0.45SiO4 between the orthosilicates Ca2SiO4 and Ba2SiO4 yields the best phosphor hosts, and interesting luminescence properties can be found from the Eu2+ singly doped and/or Eu2+/Mn2+ codoped Ba1.55Ca0.45SiO4 phosphors. The phosphors can be excited by near-ultraviolet (nUV) light at wavelengths ranging from 200 to 450 nm matching well with the nUV light-emitting diode (LED) chips. As a result of fine-tuning the activators of different Eu2+ content and Eu2+/Mn2+ couples with different ratios, tunable full-color emission under UV light excitation can be realized by combining the blue emission (460 nm) and green emission (520 nm) originating from Eu2+ with the red emission (595 nm) from Mn2+ in the Ba1.55Ca0.45SiO4 host lattice. Energy-transfer efficiency between Eu2+ and Mn2+ increases and tunable emission can be obtained with increasing Mn2+ doping content. These results indicate that the Ba1.55Ca0.45SiO4:Eu2+,Mn2+ phosphor will have potential use in nUV chip pumped white LED devices.