Chemical formula and colour of manganate ion is 

MnO₄^2-, Green

Potassium permanganate is prepared by fusion of MnO₂ with an alkali metal hydroxide and an oxidizing agent like KNO₃. This produces the dark green K₂MnO₄ which disproportionate in a neutral or acidic solution to give permanganate.

2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O

3MnO₄^2– + 4H⁺ → 2MnO₄^– + MnO₂ + 2H₂O

The manganate and permanganate ions are tetrahedral; the π-bonding takes place by overlap of p orbitals of oxygen with d orbitals of manganese. The green manganate is paramagnetic because of one unpaired electron but the permanganate is diamagnetic due to the absence of unpaired electron.