In a way that lacks color or vividness; in a dull, uninteresting manner.
From 'colourless' (adjective) + '-ly' (adverb suffix). This is a compound suffix formation: adjective + '-less' (no-ness) + '-ly' (manner).
The adverb form 'colourlessly' is rarely used in modern English, showing how some grammatical combinations are theoretically possible but practically abandoned—most speakers would say 'in a dull way' instead, preferring simpler constructions.
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