Spectrally tunable inorganic solid solutions discovered in the past few years generally cannot show spectral tuning properties without using either external rare earth (RE) (e.g., Eu2+, Ce3+) and non-RE (e.g., Mn2+, Bi3+) ions, or the coupling arrangement of these RE/non-RE ions as luminescent activator(s). Herein, we report a type of external dopant-free Sc(Px,V1−x)O4 (0.0 ≤ x ≤ 1.0) solid solution that exhibits color tuning, enhanced photoluminescence (PL) intensity, and improved thermally induced PL quenching stability. Our powder X-ray diffraction (XRD) and PL findings show that a gradual substitution of larger V5+ ions with smaller P5+ ions not only leads to the decrease of lattice parameters a(b)c and Sc–O bond lengths and the shrinkage of the cell volume (V), but also causes an initial redshift of the emission position from 495 nm to 524 nm as x increases to 0.2 followed by a subsequent blueshift back to 457 nm for 0.2 < x ≤ 0.9. Meanwhile, an enhancement of ∼40% of room temperature emission intensity at x = 0.3, along with improved quantum efficiency (QE) and thermal PL quenching after adding an appropriate amount of P5+ ions, is observed in the Sc(Px,V1−x)O4 (0.0 ≤ x ≤ 0.9) solid solutions. Through a combined analysis and discussion of the density functional theory (DFT) and dielectric electronegativity calculations as well as the UV-vis diffuse reflectance and temperature-dependent PL spectral measurements, we reveal that the origin of this unusual yet regular spectral tuning and the improved PL properties mostly arises from the interplay of the bandgap energy adjustment, the bond covalency regulation, and the possible presence of a significant amount of oxygen vacancies, together with the closing structural rigidity caused by the lattice microenvironment modification. This work can provide insights into designing and discovering new external dopant-free inorganic solid solutions with color tunability, improved emission intensity and thermally induced PL quenching behavior, and allow gaining better understanding on how the key parameters govern the spectral tunability in virgin (or doped) zircon crystal systems at the same time.