To increase the concentration of MnO ions (which are purple), you would want to shift the equilibrium to the right in the given reaction:
\[ 3MnO^{2-} (aq) + 4H^+ (aq) ⇄ 2MnO^{4-} (aq) + 2H_2O(l) + MnO(s) \]
Adding MnO ions (MnO²⁻) to the solution would increase the amount of these ions and shift the equilibrium toward the products, producing more MnO, which is purple.
Thus, the answer to your question is: MnO ions (MnO²⁻).