A couple of articles I would recommend to you.

First, this one on tartan colors:
http://www.albanach.org/colors.html

Second, this one entitled "Which tartan is mine?"
http://www.albanach.org/whichtartan.html

Then there is this one on tartan terminology:
http://www.albanach.org/terminology.html

In short, what you have here are three basic Buchanan tartans (I'm leaving aside for the moment the dress and other variants not typically woven today), all available in a variety of color schemes from a variety of mills.

You have your standard Buchanan tartan. Then you have the Old Buchanan (which as the name implies, is an older design). And you have the Hunting Buchanan.

All three of these may be woven in any number of color schemes (explained in the above articles), the most common of which are modern, ancient, weathered/reproduction, and muted.

Finally, as Buchanan is a popular tartan, most any woolen mill who weaves tartan will weave one or more versions of the Buchanan tartan, and they each have their own stock shades of yard, so that Lochcarron's "Buchanan ancient" will look a bit different from House of Edgar's "Buchanan ancient" and so forth.

And of course we have to remember that the tartan will be woven in various weights of wool for use in various applications.

And there you have it. You end up with dozens of options!

The question is, which tartan and color option do you prefer, and which weight wool is going to work best for what you plan on doing with it?