Why was the pure white coloration deliberately cultivated for the West Highland White Terrier?

To stand out clearly against dark ground and foliage

The requirement for the Westie to be pure white was a deliberate selection made by historical breeders, not for camouflage, but for visibility. In the rugged Scottish landscape where they hunted vermin, the goal was for the dogs to be easily distinguishable from the prey they might have caught. This meant ensuring the white coat provided high contrast against the dark ground and surrounding foliage. Any markings, such as black, cream, or tan patches, are therefore considered faults in the show ring because they detract from this critical uniformity and visibility standard.