In this Letter, we study diffuse reflectance and photoluminescence spectra for O(2-) fully coordinated green-emitting Ce(3+) and N(3-) partially coordinated red Ce(3+) in Ca(3)Sc(2)Si(3)O(12)(CSS):Ce(3+), N(3-) as a function of CeO(2) and Si(3)N(4) contents in the raw material. Our results indicate that the presence of N(3-) can enhance Ce(3+) solubility in the form of red centers in CSS. At low Ce(3+) concentration, green Ce(3+) forms preferentially while red Ce(3+) hardly forms even if N(3-) content in the raw material is sufficient. There exists a threshold concentration of green Ce(3+); only beyond that can color tunable luminescence with enriched red emission be achieved. Energy transfer from green Ce(3+) to red Ce(3+) is also studied, as only the green Ce(3+) is excited by blue light.