Having had color removed; stripped of color (British English spelling).
From decolourise + -d (past participle marker). In British English, the past tense and past participle of 'decolourise' simply adds -d to the stem.
The past tense of verbs ending in 'e' just needs a 'd' added in English, which is why 'decolourise' becomes 'decolourised'—this simple rule applies to thousands of verbs, making English conjugation actually quite predictable!
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