Almost 'rusting' on the ends would that sound right?
I would look at two things involving nutrition which would be worming for whipworms which can hide undetected through mutiple worm tests of stool, and looking at what you are feeding. Many coat and skin problems are best treated from 'the inside out' through nutrition. Eukanuba or Iams is believed to add brown to coats by many for example.
With a dog the coat is a last place to get nutrition so its a first place to look at condition as it will show there.
Another possibility is the way she is being groomed. Terrier type coats should have the old dead coat pulled out not clippered. I use a tool called a Mars Coat King to help with that and it pulls out dead coat nicely. Clippering and scissoring can work fine on pets but you can end up with some coat changes from doing so including getting some whispier hair with less color.
If you look at the color information in the breed standard here
You will see they list 'fading' to a matt black from a glossy black as nothing to penalize but that black and silver pattern dogs should show no browning. That tells you browning can happen.
Another possibility if the color change is only in the leg or paw area she might be licking herself due to allergies and that can cause coat to turn brown on the surface like it can near the eyes as the saliva allows an iron fixing bacteria to grow in the hair creating the 'rust' staining.
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