Wow a lot of good feed back here. To collect them:

1) This is clearly focused on the USA but as Moose pointed out has been shown to work in other places.
2) Cost for a team tartan is not a minor factor.
3) You could spice up a current tartan with you team color and logos.
4) At least two of use would already like to get colors for our team.

One thing I would like to make clear is that if we only look to current kilt makers to get colors for our teams then we are only talking to each other. We want to get other talking about and to us. Black and gold tartans are common enough that I could easily find a nice one to put a Steelers logo on. Yet even if that met some limited need once you started selling it to others you would be hunted down and shot by the NFL lawyers for creating un-official merchandise.

What I am suggesting is the creation of “official” NFL merchandise that will appear on every team web page. When a fan goes looking for a new hat or terrible towel, they will see an official team tartan. They think about getting it, and then when they go to the local team bar they see one of the fans wearing on and it goes from there.

This would also help with the cost problem since the NFL is a large company and would get a large order and economies of scale would kick in.