Awww, Sable Merles. We've got one of those. They are the chameleons of the Sheltie world.
Attached is a pic of our Lerwick at 8 weeks, and one of him at one year.
He'll be 14 months in a couple weeks, and as you can see, he hasn't settled on a color yet either.
The breeder told us he'd eventually lose his merle markings, and his coat would darken so that he would look almost indistinguishable from a regular sable. His mother, also a smerle, certainly didn't look much different from the other sables. Well, so far the breeder is only half-right. Most of Lerwick's merle markings are gone, though he does still have a big one at the base of his tail, and one on the back of his neck, in his ruff... but he is still definitely lighter than your average sable... definitely a ginger as opposed to an auburn color.
Like ClantyreSheltie said... smerles go through some drastic changes, even after they're finished growing. It will be interesting to see how MacGreggor changes as he matures. He sure is adorable, and that will always be true, no matter what color he settles on!