1 Answer
No, if you boil thistle flowers at 212°F, the resulting solution will turn light brown.
It makes okay light brown water-color paint
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1There are a lot of articles on natural dyes, such as this Wikipedia page but none mention thistles. Some colours just aren't stable enough. Safflower is a yellow thistle that produces a nice yellow dye, but it prefers much warmer weather than Scotland.– Stuart FCommented Apr 25 at 9:55