The Wild Scots' Are clothed after the Irish fashion, in striped mantles, with their hair long and thick.
-1607, Camden, in his Britannia.

Lady Montgomery, wife of Sir Hugh Montgomery, 'set up and encouraged linen
and woollen manufactory (in Ulster), which soon brought down the prices of the breakens (tartans) and narrow cloths of both sorts.' The beginning of such (manu-)factories might be part of the reason woollens were replacing the linen shirts, brats and animal skins around the turn of the century.
-1613

It appears that the desire for uniformity in the colours of tartan used by a clan was beginning in the early 1600's: "remove the red and white lines from the plaides of his men so as to bring their dress into harmony with that of other septs.
-1618, Letter from Sir Rbt. Gordon of Gordonstoun to Murray of Pulrossie

"Many Highlanders were observed in this town (Leith), in their plaids, many without doublets, and those who have doublets have a kind of loose flap garment about their breech, their knees bare. The inure themselves to cold, hardship, and will not diswont themselves. Proper personable well-completed men, and of able men: the very gentlemen in their blue caps and plaids."
- 1635, Sir William Brereton.

Lowland Dress

The husbandmen in Scotland, the servants, and almost all in the country did wear coarse cloth made at home, of grey or sky-colour, and flat blue caps, very broad. The merchants in cities were attired in English or French cloth, of pale colour, or mingled black and blue. The gentlemen did wear English cloth, or silk, or light stuffs, little or nothing adorned with silk lace, much less with lace of silver or gold, and all followed at this time the French fashion, especially at court. -Englishman Fynes Morison, visiting Scotland 1598 (Whalebone sleeves: sleeves stretched on whalebone hoops. Falling bands: A deep linen collar, turned down.)
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CONTEMPORARY QUOTES CONCERNING SCOTTISH WOMEN'S ATTIRE

Women's Highland Dress

"The dress of the women among them is most becoming, for over a gown reaching the feet, and very richly adorned by the Phrygian art (embroidery), they wear very full cloaks, of several colours, such as I have described - loose and flowing, yet gracefully drawn into folds, as they will. With their arms tastefully adorned with bracelets, and their throats with necklaces they have great grace and beauty."
-Bishop Lesley, 1570's. The original is in Latin, and uses the word tunica, for gown, which may suggest a straight-hanging fullness of more Medieval style, in contrast to the more fashionable farthingale.

Women's Fashions, Edinburgh

The original paragraph has been broken up by social class to help make the descriptions distinct from each other.

"The women here wear and use upon festival days six or seven several habits and fashions, some for distinction of widows, wives and maids, others apparelled according to their own humour and fantasy. Many wear (especially the meaner sort) plaids, which is a garment of the same woollen stuff whereof saddle cloths in England are made (A close felt-like cloth the would keep out rain), which is cast over their heads and covers their faces on both sides, and would reach almost to the ground, but that they pluck them up and wear them cast under their arms." Some ancient women and citizens wear satin straight-bodied gowns, short little cloaks with great capes, and a broad bonegrace coming over their brows and going out with a corner behind their heads: and this bonegrace is as it were lined with a white starched cambric suitable thereto." (Bonegrace: a silk, or cloth hood over a starched under-coif projecting around the face like the headgear of some religious orders?) "Young maids not married all are bare-headed, some with broad thin shag ruffs, which lie flat to their shoulders, and others with half bands, with wide necks, either much stiffened or set with wire, which come only behind: and these shag ruffs, some are more broad and thick than others."
- 1635, Sir William Brereton.

Bands with wide necks seem to be the broad lawn collars on each side of a square décolletage, as in the paintings of Van Dyck. These seem to have reached Scotland sooner than England. Van Dyck's portrait of Mevrouw Leerse shows this collar, with the tilted back cut separate, and edged in lace. It
is shown with deep cuffs to match on a black satin dress. His portrait of Marie-Louise de Tassis has another, with the back part pleated. Later the stiffening went, and it lay flat. The "shag ruff" is a puzzlement. According to the Oxford Dictionary, shag was cloth of wool or silk, with a velvet nap
similar to a modern velour. The true ruff was of linen, perhaps with lace, and did not lie flat. The author may be describing a pleated tippet, worn for warmth above the low-cut dress of the day.

Lowland Dress

The original paragraph has been broken up by social class to help make the descriptions distinct from each other.

Gentlewomen married, did wear close upper bodies, after the German manner, with large whalebone sleeves, after the French manner, short cloaks like the Germans, French hoods, and large soft falling bands about their necks. The unmarried of all sorts did go bareheaded and wear short cloaks with most
close linen sleeves upon their arms, like the virgins of Germany. The inferior sort of citizen's wives and the women of the country did wear cloaks made of coarse stuff, of two or three colours of checker-work, vulgarly called ploddan. To conclude, in general they would not at this time be attired after the English fashion in any sort, but the men, especially at court, followed the French fashion, and the women, both in court and city, as well as in cloaks as naked heads and close sleeves on the arms and all other garments follow the fashion of the women of Germany.
-Englishman Fynes Morison, visiting Scotland 1598 (Whalebone sleeves: sleeves stretched on whalebone hoops. Falling bands: A deep linen collar, turned down.)

Sources:
The Scottish Pageant 1513 - 1625. MacKenzie, Agnes Mure. Oliver and Boyd:
Edinburgh & London, 1948.

The Scottish Pageant 1525 - 1707. MacKenzie, Agnes Mure. Oliver and Boyd:
Edinburgh & London, 1949.

A Short History of the Scottish Dress.Grange, R.M.D. Burke's Peerage
Limited: London, 1966.