My experience is limited...more based on what I've read than experienced. Sounds like a larger darker fly (all the way to black)so that it can be seen against the dusky night sky from below by Mr. Coho. Also, one that pushes more water to create sound waves - like a modified deceiver with a larger head - a Sea-ducer is one example http://www.nesportsman.com/nh/jimsflybox/fly4.shtml. I wonder of a popper might work as well. My 2 cents
I've fished for Coho during dusk (and well into dark) hours on many occassions and have been quite successful.
I know you are well aware game fish use sight, sound, smell and even impulse (vibration) to locate prey. Regardless of the sense used to locate the prey, the final sense that comes into play is sight; they see it, they attack it. Game fish have both rods and cones in their eyes and, as light diminishes, their vision shifts to predominately black and white which basically maximizes visualization through contrast. A fly that creates turbulance, such as Dan Blanton's Whistler, might work well to draw their attention during these times, since it creates turbulance as its retrieved. Surface poppers and sliders could also work well as they create disturbance on the surface. In both cases, you're using a fly that gets their attention by creating commotion which draws them near at which point their vision takes over and, whether they strike through hunger, anger, territorial protection or ignore the fly totally, at least they are near the fly.
Bass fishermen (and many Steelhead fishermen for that matter) maintain a dark bait/lure during low light works well since the dark colored object creates a better silouette against the low light background when viewed from the fish's perspective. This would certainly reinforce Surf_Candy's recommendation for the dark fly during low light.
I could probably change the odds more into my favor by using flies that create commotion or are dark in colour, but the success I've had has been using traditional ties (e.g. Clousers, Sand Lance etc.)but then again, I'm old enough to be set in my ways. Let us know how you approach it.
greg have you ever caught a salmon on a fly after dark? I have never seen a salmon taken after dark on a artificial before except 2 inch white worms fished under lighted piers casted to schools. I am an avid night fisherman and salmon on artificials at night have never seen or heard of before except that situation. but herring under a bobber and casting cutplug herring to fish under lights at night is probly the most successful salmon fishing from shore if done right that I have ever seen and done. Ben