Border Collies fur color is changing?

I have a 4 month old Border Collie named Milo. About a month ago i noticed that he had gotten a little bit of brown fur in his ears. Than last night i was brushing him and i noticed that his back legs are turning brown. He has white feet and above the white fur it's turning brown. This morning it was a really light tan color but now it's a darker brown. Also on the left side of his muzzle he's getting brown fur about the white fur. Does this mean he's going to get more brown fur? I actually always wanted a tri colored Border Collie and i'm pretty excited for more brown fur to come!(: