Irregular pattern of indistinct light and dark areas

Merriam-Webster Online Dictionary
mottle (noun)
a colored spot
a) a surface having colored spots or blotches
b) the arrangement of such spots or blotches on a surface
- mosaic
mottle (verb)
transitive verb
to mark with spots or blotches of different color or shades of color as if stained
mottle (Wikipedia)
This Appaloosa horse shows mottling around its mouth and nostrils. In this particular case, it is a normal hereditary trait linked to a spotted coat color pattern and not a sign of disease

Mottle is a pattern of irregular marks, spots, streaks, blotches or patches of different shades or colours. It is commonly used to describe the surface of plants or the skin of animals. In plants, mottling usually consists of yellowish spots on plants, and is usually a sign of disease or malnutrition. Many plant viruses cause mottling, some examples being:

Mottling is sometimes used to describe uneven discolored patches on the skin of humans as a result of cutaneous ischemia (lowered blood flow to the surfaces of the skin) or Herpes zoster infections. The medical term for mottled skin is dyschromia. Although this is not always the case, mottling can occur in the dying patient and commonly indicates that the end of life is near. Mottling usually occurs in the extremities (lower first) and progresses up as cardiac function declines and circulation throughout the body is poor. In animals, mottling may be a sign of disease, but may also be a hereditary trait, such as seen with the champagne and leopard complex genes in horses.

Mottles can also refer to discoloration in processed food, such as butter.

In geology, mottled refers to a patchy/blotch texture of alteration or interbedding, commonly found in limestone and commonly caused by bioturbation.

Mottling can also refer to an undesirable defect which can occur with effect coatings, most obvious on light metallic finishes. The total color impression shows irregular areas of lightness variations. These "patches" are usually visually evaluated, described as a mottling effect. Some also feel that it reminds them of clouds. This effect is especially noticeable on large body panels. It can be caused by the coating formulation, as well as variations in the application process. For example, disorientation of the metallic flakes or film thickness variations of the basecoat can lead to various mottle sizes resulting in a non-uniform appearance. The visual perception of mottling is dependent on the viewing distance: Large mottles can be seen in far distance evaluation, while small mottles are more noticeable in close up evaluation. The visual evaluation of mottling is very subjective, as it depends on the illumination conditions, the observing distance and the viewing angle.

In graphics printing mottling refers to an uneven coloration resulting from letterpressed printing of textured papers, mainly in larger colored surfaces. Due to the uneven surface, not all fibers of the paper are evenly saturated with color unlike offset printing.

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