Then I accidentally stumbled on the answer. Reading Jude's post about dyeing corners of cloth with indigo, I thought I'd try putting a wet, used decaf Twinings English breakfast tea bag on the corner of a too-bright yellow patch of linen to see if it would leave a triangular mark. It didn't.
What the tea did do was migrate through the entire patch of linen. And somehow, as it dried, the color concentrated along the edges ...
I liked the effect so much that I stained three more yellow linen patches, this time using Tazo Zen green tea bags, which left lighter marks, but still pleasing. So I stitched the four patches together using a wonky variegated gold/brown/rust floss that has never before worked visually. This time it did ...