Yttrium tantalate, yttrium niobium-tantalate, and yttrium niobate doubly doped by Eu3 and Tb3 were investigated using X-ray diffraction and X-ray excitation luminescence in order to study their structural and luminescent properties. By means of X-ray diffraction, the crystallographic data for YTaO4, Y(TaNb)O4, and YNbO4 with double activation by Eu3 and Tb3 was first calculated. Under X-ray excitation luminescence, the rare earth emission centers can contribute to the overall luminescence. The simultaneous incorporation of Eu3 and Tb3 ions could permit us to obtain different luminescence colors on the entire visible spectrum. Due to their various luminescence chromaticities, the proposed rare earth activated phosphors are promising materials for optoelectronics as well as for X-ray intensifying screens for medical diagnosis providing the broad variation of photoluminescence colors from blue-to-green to yellow-to-red.