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Pearsall's Gossamer No. 10 (Ash) is roughly matched by Gutermann's Color 586 (unfortunately missing the slightly tan hue)
Pearsall's Gossamer No. 11 (Olive Green) is closely matched by Gutermann's Color 715
Pearsall's Gossamer No. 16 (Olive) is closely matched by Gutermann's Color 775
Note - For those purists out there, the Gutermann's threads shown above are 100% polyester threads, as compared with Pearsall's 100% pure silk. The photo above shows the color comparison when the threads are dry. The color may differ when the threads are wet. I would think that the color is close enough if the thread is to be used for flymph bodies (partially obscured by dubbing).