End of September 2019, a mate and I were taking a fish each and swapping over, out of a shoal of Grayling that were taking something we couldn’t decipher. The pattern Martin got his first one on was a #16 cdc Emerger, just olive thread but on a specific hook, I changed to the same and as long as we bullied the caught fish away from the shoal, they were not spooked, the released fish seemed to go right back among them. I decided to try a pink Emerger, on the same hook, I caught, Martin tried yellow owl on same hook, caught. I tried the same olive as the original, on a #b160 #14 which was similar size but very different shape, nothing.
Martin moved back in with the original fly, fish straight away.
We fkkd around a bit with other sensible patterns, but they wanted that shape, and wanted it drag free.
Those were the 2 essential components, colour did not matter at all.