We study from first principles two lanthanum silicate nitride compounds, LaSi3N5 and La3Si6N11, pristine as well as doped with Ce3+ ion, in view of explaining their different emission color, and characterizing the luminescent center. The electronic structures of the two undoped hosts are similar, and do not give a hint to quantitatively describe such difference. The 4f→5d neutral excitation of the Ce3+ ions is simulated through a constrained density functional theory method coupled with a ΔSCF analysis of total energies, yielding absorption energies. Afterwards, atomic positions in the excited state are relaxed, yielding the emission energies and Stokes shifts. Based on these results, the luminescent centers in LaSi3N5:Ce and La3Si6N11:Ce are identified. The agreement with the experimental data for the computed quantities is quite reasonable and explains the different color of the emitted light. Also, the Stokes shifts are obtained within 20% difference relative to experimental data.