Uniform nanorod-like Eu2+, Tb3+, Cr3+ co-doped non-stoichiometric Mg–Al spinel was synthesized via hydrothermal treatment (140 °C × 24 h) followed by mild calcination at 1100 °C. The effects of stoichiometric ratio ( \(n_{{Mg^{2 + } }} :n_{{Al^{3 + } }}\) = 1:x) on Tb3+ single-doped non-stoichiometric spinel were investigated, along with the energy transfer and multicolor luminescence regulation in Eu2+, Tb3+, Cr3+ co-doped samples. Results show that: As the Al stoichiometry increased from x = 2.0 to 3.2, the Tb3+ single-doped samples transformed from a mixture of aluminum-rich spinel and MgO phases into single-phase aluminum-rich spinel. At x = 4.0, the defect-rich aluminum-rich spinel structure exhibited the most significant enhancement effect on Tb3+ emission. Building upon the optimal Tb3+ single-doped system, the introduction of f-d transition ions (Eu2+ and Cr3+) formed effective co-doping without altering the host structure. Under 339 nm excitation, the Eu2+ → Tb3+ energy transfer efficiency reached 65.09%, enabling color-tunable emission from bluish green → pale blue → pure blue in Eu2+, Tb3+ co-doped samples. By varying excitation wavelengths between 330 and 377 nm, the Eu2+, Tb3+, Cr3+ tri-doped system achieved multicolor modulation from pure blue → pale blue → ultimately near-white light. This phosphor is therefore a promising candidate for multicolor display applications, especially for white-light emission.
