A substance or agent that removes color; a bleaching agent (British English spelling).
From decolorise + -r (British agent noun suffix). The -r or -er suffix creates nouns for things that perform the action described by the verb, following patterns like 'maker' from 'make'.
English's -er suffix is one of its oldest and most productive ways to name things that do actions—it's so automatic that we rarely notice we're using an ancient pattern inherited from Old English and Germanic languages!
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