A picture would have helped a lot, upload a pic so people can have a better idea.
i could not give you a good answer, first time GSD owner myself, but i will tell you about my own experience (very limited one) that it does darken up a bit...and sable tend to change their coat and colour but my Tyson who is Blanket back black and tan darken up a bit between 3-4 months.
By 5 months he had shed all his puppy coat and changed to shiny black and tan.
Edit; i think i just made you more confused
what can i say..it's a gifted talent.