I'd go right to Old Sturbridge Village and ask them. They dye a lot in the summer there using natural dyes and I think the colors I have seen there could get a nice range of tartan colors. By the way; a nice saffron color can be gotten from onion skins from what I remember. I have not checked their website - just recollecting many, many times at the Village in the past 45 years. Would be fun to follow this and see what we can find.