Use 'mottled' to describe something when you want to create a vivid, visual image in the reader's mind. She admired the mottled shades of the marble.
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Not Uniform
'Mottled' suggests an irregular or uneven pattern, not a consistent, uniform one. The artist's palette was mottled with paints.
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Natural Description
It's commonly used to describe natural patterns, like on animals, plants, or landscapes. The mottled leaves of the tree camouflaged the chirping cicadas.