Yes, hair color can definetly change as puppies develope, think of how a Yorkie is mostly black as a puppy. What's confusing me is that Border terriers are not white or blonde, they are brown or brindle, if I recall. Also, they aren't supposed to have "long hair" like a Maltese or Shih-Tzu. Their coat should be around 1.5-2.5", and it should lye tight against her body. Is she possibly a mix?
Many groomers buy human products to dye their animals bright neon colors for creative-grooming competitions. Manic Panic is a very popular brand. But, they are never dyeing to enhance natural markings. They are coloring things like a pink flower on the dog's hip, for exapmle.