I can't seem to get fly design out of my head...
Use the right flies for winter steelhead and catch more? Some guides will tell you the fly doesn't matter. When a steelhead wants to eat your fly it will. I like to optimize the visibility of my fly and consider other environmental factors when it comes to fly selection.
First, bright fish in warmer water will hit most any fly. Give them a highly visible pattern in green water that has movement and draw. As water drops and clears fish move more slowly into a river system. Darker fish that have been in the river a while are often the only ones around. Low clear cold conditions slow down migration. For these fish that have been there a while go with darker patterns. Black/blue, black/red, purple are more likely producers in this water scenario.
What ever fly you choose, pick one you are confident in. A fly that has movement and profile for winter waters are often best producers.