When I was a kid, there was a study about painting firetrucks day-glo yellow because red can appear brown at twilight, and that the yellow sticks out like a sore thumb.
Hunters, of course, wear orange.
I imagine that any extremely bright color would help, and if it doesn't appear in nature, that probably enhances it.
I have to tell you, while following a guy on the racetrack back around Labor Day, I noticed something weird about his bike - it seemed to have an orange puddle that followed it everywhere. Turns out it was his lower fairings reflecting orange light onto the track. Yes, they were bright. And you certainly couldn't miss the bike.