Dark imitates newly emerged rainbow fry and the light mimics whitefish fry. BTW, fish LOVE to eat whitefish fry.
Dead drifting a streamer is deadly effective. I designed this fly to be fished in tandem with a stonefly or green drake nymph as an anchor fly. cast upstream, dead drift it and then swing it towards shore. HOLD ON! takes can happen anywhere along the drift. Stout tippet is recommended.