Dear student,
Citric acid turns potassium permanganate from purple to colourless because it is a reducing agent.
KMnO4 will oxidise citric acid to CO2 and H2O and itself will turn into almost colorless Mn2+ (in acidic environment), or into brown MnO2 precipitate.
In words we can say that addition of citric acid to potassium permanganate decolorises the purple solution yielding Manganese(II) ions, carbon dioxide and water.
The chemical reaction is given by:
Regards.