Dog coloring?

From those cute wonky ears, he looks about 3 or 4 months old. His color will change for another year before it settles on what he's going to look like. The gray stripe will always be there. Expect him to get more of the grays in his back.

Undercoat showing through?

I was hoping mygirl would stay as black as she is...but in the sun I can see she's going brown in places. They like to play the color morph game.

His coloration is perfectly normal and doesn't mean anything. A lot of Shepherds have a lighter stripe behind their front legs and shoulder and usually their bum fur (or "pants") are light colored as well. Grey or lighter colored hairs within the black saddle is normal as well.

Since your puppy is still pretty young, his coloration is going to change quite a bit as he gets older. The black area is probably going to look much smaller when he's fully grown versus what it looks like now. He'll likely also have more of the lighter fur in the black around his neck and along his back.

Totally normal coloring! Handsome puppy, too!