When I decided to write a Scottish medieval novel, I discovered that my "idea" of what a medieval highlander would wear was completely incorrect. What I had seen in photos and movies like Braveheart and had read in other Scottish novels had given me the wrong impression. Even after I discovered this, my opinion of the movie didn't change. I still love it. But the depiction of the characters in my book would be different.
The Great kilt or Belted Plaid dates back to 1594. The great kilt was an untailored garment made of cloth gathered up into pleats by hand and secured by a wide belt. The upper half could be worn as a cloak draped over the left shoulder and secured by a brat (clip) or draped down over the belt and gathered up at the front. In cold or wet weather, they might have brought it up over the shoulders or head for protection against weather.
Before the Great kilt or belted plaid, they wore a long shirt that is known as a "leine" in Gaelic and thought of as a "tunic" in English. A plaid of wool cloth would have been draped over the shoulders and around the arm and fastened by a brat. The tunic came down to the knees on a man and was much longer on a woman. Because of the length on a woman it was similar to what we think of as an English chemise.
The association of clan family specific tartan colors and plaid designs was a late development in the 17th & 18th centuries. However, much earlier family clans that lived within a region would wear similar plaids and colors because they used the same seamstresses in the area. And of course, families that intermarried typically lived in the same region in medieval Scotland, especially in the highlands. Much of the clan colors and design patterns associated with specific family clans probably derived from this regional practice.