The fact that anhydrous transition metal salts are colored is a well-established property arising from d-d electronic transitions, requiring no citation.
The claim that anhydrous transition metal salts are colored is a standard chemical fact taught in basic chemistry, rooted in crystal field theory and d-d electron transitions. None of the retrieved papers specifically test or focus on this broad introductory chemical rule as a novel hypothesis, making it a piece of common knowledge rather than something requiring bibliographic verification from this specific set of literature.