Dark colors that make noise....or push water......or have big profiles, or all of the above. Having said that, a bass will find most flies, if it's in their zone. Last time I checked, small shad/minnows do not wear cow bells around their neck. Bass eat baitfish at all hours of the day and night. Increasing the odds with sound, or vibration can be a good thing though.
Try not to shine light on the water. If your tying on a fly, shine the light away from where your fishing, or use a lower intensity, red lens light(I use a headlamp with that option). Try not to watch Jaws the night before. It gets creepy out there in the darkness.
I have found a "dead period" when the sky goes from dusk, to black. That transition period can be less than an hour, up to a couple hours. Sticking with it can pay off.
Oh, and one more thing. If you have casting flaws(who doesn't), you might be surprised how not being able to see your flyline will effect your cast........negatively.
briansII