There are a few possible explanations for this:
* Genetics: The Westie's white coat color is a dominant trait. This means that even if a Westie has a black gene, it will not be expressed in its coat.
* Breeding Standards: The breed standard for West Highland White Terriers explicitly specifies a white coat. This is part of the breed's defining characteristics and any dog that does not meet this standard would not be considered a purebred Westie.
* Color Variations: Some Westies might have a slightly darker coat color than others, but this is usually due to environmental factors, such as sun exposure or aging, and not a true color variation within the breed.
It's important to note: If you see a dog that looks like a Westie but has a black coat, it is most likely a mixed breed.