Hydrogen peroxide is used to restore the white colour which was lost due to the reaction of hydrogen suiphide in air with the white pigment Pb3(OH)2 (CO3)2 to form black colored lead suiphide (PbS) Hydrogen peroxide oxidises black coloured lead suiphide to white coloured lead sulphate, there by restoring the colour.
PbS + 4H2O2 → PbSO2 + 4H2O