Q/: The transition metal ions having empty or complete d orbitals are colorless, but why KMnO4 is intense purple?

The fact that “transition metal ions having empty or complete d orbital are colorless” only applies to simple monatomic ions such as Sc³⁺ ion (empty 3d orbitals) and Zn²⁺ ion (complete d orbitals).

Mn⁷⁺ has empty d orbitals, but Mn⁷⁺ does NOT exist due to its extremely high charge density.

MnO₄⁻ is a polyatomic atomic ions, but NOT a monatomic ion. MnO₄⁻ has a resonance system containing three Mn=O double bonds and one Mn-O⁻ single bond. The deep purple color of MnO₄⁻ is because of the absorption of light to promote electrons to the higher energy levels of the resonance system.