Dark streamers as a black zonker or a muddler, on the right, are especially deadly at night when the bigger fish feel confident to leave the deep basins and start to hunt in the shallower waters. If you want to catch the bigger trouts, that is the way to go.
Streamers can also be used daytime to fish the deep holes and backwaters but then in lighter colours.
This muddler, on the right, can imitate a big caddisfly, if you fish it as a dry fly as well as a fish if you let it sink and fish it as a wet fly. A very versatile pattern that can be tied in a lot of sizes to suit the conditions. One of my favourites for big grayling.
On the left, a nice grayling taken in northern Sweden. In the middle of the night you have a chance to come in contact with the bigger fish that use the dark hours to feed. This one wasn't an exception and took the black rabbit zonker when retrieved against the current.