I am reading a textile history book (The Fabric of Civilization: How Textiles Made the World by Virginia Postrel) (very good, highly recommended) and about 20 minutes ago I learned that one of the bushes in my front yard, that I have to trim regularly, was an important yellow dyestuff in medieval Europe
so for the past 20 minutes I have been searching the Internet and consulting with @ratbabyonline on how to make my own yellow dye. keep watching this space.
I am currently unable to find anything about time of year for harvest for optimal dye load (google turns all "fustic" into "rustic" and even then most of the "fustic" results are for "old fustic" aka a completely different plant) so this might require a lot of experimentation
maybe you’ve already tried the scientific name but perhaps that could help? experimentation is fun though! builds a deeper relationship with the plant and its seasonal life cycle
Unfortunately yes, searching with the scientific name (Cotinus coggygria, prev Rhus cotinus) or other common names seems to just give me breathless writeups about how good it looks in gardens (true, I can attest to this) with maybe a side historical note about how it used to be used as a dyestuff. 
it seems as if you’re entering the realm of forgotten arcane knowledge and the rediscovery is up to u if you so choose
yeah my next step is the university library and after that it's Just Go For It
