It shouldn't be a "line" per say. The breeder shouldn't let someone just show up and pick a puppy. That's pretty much just a choice based off color as no one is going to get to spend more than an hour with that litter while the breeder has spent weeks with it.
The breeder should be placing dogs based off of their temperaments, drives, ect into correct homes for that dog depending on what the owner wants out of that dog.
In your situation...the "first pick" could show up, want a dog for a nice, quiet family life with little obedience work and little day to day activity and end up with the most energetic, driven puppy because they fall in love with its color. Second pick could be someone looking for an IPO or other sport dog, but also fall in love with a color which then happens to be the calmest, not want to do anything dog.
CD RE TC HIC 7/10