I think it all depends. Jackson started changing the most after his second haircut. Suddenly all the black puppy fur was gone!

But he still seems to changing, in smaller ways, at over 2yrs old... every time he comes back from the groomer, I swear he is a different color, LOL!! But I've seen some Yorkies who stay their dark black/tan all their lives.
First pic-9 weeks
Second pic-4 months-ish
Third pic-1 year
Fourth pic-2 yrs