I asked basically the same question to my vet the other day. Mainly concerning the sheen of the coat. He's still a big furball and his puppy coat is so luxurious but very flat in sheen. The vet said , "no problem", at about 6 months the double coat should start to form and it's generally the long hairs that shine so well.
I know that wasn't exactly what your question was but as I recall with past GSDs they seemed to be in full color by about 1 1/2 to 2 years.
It's fun to watch now as each week goes by and some shading of red pops through from the predominantly black coat around the muzzle, eyebrows, ears, neck etc. Mack is 2 1/2 months old.