Sulphur(IV) Oxide bleaches coloured substances through the process of reduction. It removes oxygen from coloured substances and makes them colourless, but this change is reversible as atmospheric oxygen can re-oxidize the colourless products, causing the colour to return. 

In order to get a permanent bleaching action, Chlorine is recommended. This is because chlorine reacts with water to produce nascent oxygen, which acts as an oxidizing agent and permanently oxidizes coloured substances.