A new approach has been proposed and validated to modulate the emission spectra of europium-doped 12CaO·7Al2O3 phosphors by tuning the nonradiative and radiative transition rates, realized by controlling the sort and amount of the encaged anions. A single wavelength at 255 nm can excite simultaneously Eu2+ and Eu3+ centres to yield blue and red emissions, respectively. The amount of the anions in the nanocages, like OH− and H−, can be controlled by heat treatment processes. The existence of encaged OH− accelerates the nonradiative process of the emission centres, while the presence of encaged H− induces a higher symmetry that decreases the radiative transition rate, confirmed by the analysis of decay processes. It is demonstrated that the emission colour is tuned finely from red to blue with the CIE chromaticity coordinates from (0.630, 0.352) to (0.216, 0.086) when the annealing time increases. This strategy can be readily extended to similar systems with other rare earth active centres.