I'm not trying to hijack this thread, but dyeing wool using natural or plant dyes is kind of archaeological because it has been done for centuries, e.g. info on this site:

dyes from plants

Thinking about the yellow and burnt orange/brown colours in that Tartan piece found in a bog:

Some of my best results came from coreopsis, which is a roadside weed here in Oz but I just discovered this week while watching a UK TV show called Love Your Garden that it grows in the UK. I suspect it may have been brought out here to plant in gardens and then it escaped into the wild. I also managed to get a beautiful dark burnt orange type of red from it.

Another excellent and reliable source for yellow and burnt orange/brown dyes is brown onion skins. The trick is that if you want yellow you put the untreated homespun wool yarn in the pot early in the cooking time, but the longer you boil the onion skins before putting the wool in, the more burnt orange/brown it gets.

I had a lot of fun experimenting, back in my younger days.