Why in the world is this puppy green?

(CNN)Look at this good dog! This is Rio, and Rio recently had a litter of puppies. Among the freshly-minted additions, one looked a little, well, mintier than the rest.

Rio and her owner, Louise Sutherland, live in Golspie in the Scottish Highlighands. When Rio gave birth to a litter of nine golden retriever puppies on July 7, Sutherland was surprised to find one puppy came out sporting an unmistakable shade of green.
"It was all hands on deck, but then as the puppies started to arrive," Sutherland told the Daily Mail, "we noticed that one of the puppies had green fur! We couldn't believe it."
Sutherland named the little gem Forest.
    The rare phenomenon is thought to happen when light-colored puppies come in contact with biliverdin, a green pigment found in bile. It's the same pigment you can see when bruises turn green. It essentially dyes their fur in the womb, and the color fades over time.
    In fact, Sutherland told CNN that Forest's unusual verdigris has already faded significantly. Here's little Forest a week after birth:
      And here's Forest now. Sutherland says he still has a little tint when sidled up next to his brothers and sisters, but she expects Forest will be completely de-greened soon.
      Sadly, one of the nine puppies passed away after birth, but Sutherland says the rest of the brood, including little Forest, are doing well.