To remove color from something, typically used in British English (American English uses 'decolorize').
From de- + color + -ise (British verb suffix from French -iser). The -ise ending is the standard British formation for verbs, parallel to the American -ize, both ultimately from Greek -izein.
British and American English speakers are actually saying and meaning the exact same thing when they use 'decolorise' vs 'decolorize'—it's purely a spelling convention that comes down to which side of the Atlantic you learned to write!
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