This is not a grayling fly, but a wet trout fly. Especially targetted for a river I know that has a good population of sizable brownies. They seem hidden for the whole day, especially in the high summer, but become alive at dusk when the big caddis flies hatch.
With the low light the color of your imitation is becoming less important, and what matters is the general shape and movement of your fly.
Especially effective is the "rising caddis style" - a lightly weighted imitation fished sunk across the stream, with a gentle lift at the end of the drift. That is the moment when a violent strike is supposed to happen :)
#10 Kamasan B110 grubber hook, debarbed
a layer of flat lead
yellow 6/0 Danville's tying thread
body from Jamieson's of Shetland wool, color Wren
4 strands of pearl Flashabou
3 CDC feathers for wing
a light touch of Fox Squirrel dubbing mixed with Hare