My own clan, Grant, is one of the few where any hint exists of there being any specified tartan for the clan prior to the 19th century. In 1704 the Chief specified that his men were to wear "red and green broad sprigged" when serving him.
I would cuation against reading too much into this quote. Not to imply that you are guilty of this yourself, but I have seen too many people use this as "evidence" that the Grant tartan dates to 1704, or at least the Grants had some "clan tartan" in 1704.

What does this quote really tell us?

Number one, it tells us that the members of Clan Grant were not wearing a uniform tartan to begin with, or else the cheif would not have to specify that they were to wear these specific colors when they were called to the field.

Number two, it does not mention anything about a specific sett or pattern. It just asks for red and green colors, in broad stripes. This describes about half of the tartans in common production today! So this can't be read to refer to any particular tartan. Furthermore, it would seem that the clansmen were responsible for obtaining their own cloth in these colors, so I can only imagine that there was great variety among them.

Number three, it presupposes that there were no named tartans at the time, becuase if tartans did have names, he would have said, "wear the proper Grant tartan" or something like that. He would not have had to describe the colors.

In short, all we really can tell from this statement is that the cheif of the clan wanted the men fighting under him to all be outfitted in similar colors when on the field. It was an early attempt to enforce some uniformity in military dress, and for that reason is important. But it doesn't really say anything about named clan tartans existing at the time.

Aye,
Matt