In the reaction between oxalic acid and potassium permanganate, the colour change is due to the reduction of permanganate ions (MnO4-) to manganese(II) ions (Mn2+). This reduction is accompanied by the oxidation of oxalic acid (H2C2O4) to carbon dioxide (CO2) and water (H2O).

The overall reaction can be represented as follows:

5H2C2O4 + 2KMnO4 + 3H2SO4 → 10CO2 + 8H2O + 2MnSO4 + K2SO4

At the start of the reaction, the solution is purple due to the presence of permanganate ions. As the reaction proceeds, the permanganate ions are reduced and the colour of the solution changes to colourless or pale pink. The disappearance of the purple colour indicates that the reaction has reached completion.

The colour change in this reaction is a useful indicator of the progress of the reaction and can be used to determine the endpoint of the titration.