Tartan is a particular weave of woolen cloth, which in modern times has become lumped into the category of visual patterns we have dubbed as plaid. This refers to the precise number of threads of each color used, and the order of their appearance. A sett may involve a few dozen or a few hundred threads, but the same count and colors repeat in both directions over the width and length of the piece. Another tartan matter of importance, aside from the specific kind of weave, is how they are used by the clans.
Some clans claim several different setts, each for use under certain circumstances.
The differing appearance is simply explained, and is not something elected by design. It is a matter of the fact that old dyes used for yarn are want to fade. A tartan looks one way when first created, and another way after some bit of wear and washing. We ain't sure exactly why this is the case. Scottish Austins have long been associated with the Keiths, as allies and as a sept of the clan.