One gas bleaches the colour of flowers by reduction, while the other by oxidation, the two gases respectively are;

Aqueous solution of SO2, acts as reducing agentSO2+2H2O⟶H2SO4+2[H] thus SO2in presence of moisture is used as bleaching agent. This is due to the reducing nature of SO2Coloured compound + [H] → Colourless compound Bleaching action of Cl2, is due to oxidizing nature