When sulphur dioxide (SO₂) reacts with an acidified solution of potassium permanganate (KMnO₄), the initially purple solution turns colourless. The reason is because the reaction between SO₂ and KMnO₄ is a redox (reduction-oxidation) reaction. SO₂ acts as a reducing agent, meaning it causes the reduction of another substance (KMnO₄), While the permanganate ion (MnO₄⁻) in KMnO₄ is reduced to manganese(II) ions (Mn²⁺), which are colourless. 

SO₂ is oxidized to sulphate ions (SO₄²⁻), which are also colourless. So, the purple colour of KMnO₄ is due to the permanganate ion, and when this ion is reduced, the solution becomes colourless.