A STARZ Original Series From Executive Producer Ronald D. Moore: Outlander

[Calgacus]

I also seem to recall that what few written accounts were made, commented upon the tartans as also blending in with the local plants, and thus acting as a form of camouflage.

There is a written account, I think from some time in the 17th century, where the narrator states that previously, bright tartans were in vogue but at the time of the narration duller hues were more popular, as a form of blending in or camouflage, however the evidence shows that by the time of Culloden, rather brighter tartans were the norm again.

*Edit* Account found-

In James Aikman's 1827 translation of George Buchanan's 1581 "History of Scotland":

"They delight in variegated garments, especially stripes, and their favourite colours are purple and blue. Their ancestors wore plaids of many colours, and numbers still retain this custom but the majority now in their dress prefer a dark brown, imitating nearly the leaves of the heather, that when lying upon the heath in the day, they may not be discovered by the appearance of their clothes; in these wrapped rather than covered, they brave the severest storms in the open air, and sometimes lay themselves down to sleep even in the midst of snow."