I often use two colors, some show up better in different lights.
I also find a nearby white or yellow lane marker, a stripe painted down the side of the road or a stripe in the center. Then, with dark paint, I put a couple of bars across it.
This helps me locate the vicinity of the real nail.
Since the highway department was nice enough to put down reflective paint, my small dark marks on their stripe show up wonderfully.
When possible one of us goes out the night before to bang in wooden stakes on S,F and marks. We paint the stakes flourescent orange, and staple a bit of yellow corrugated plastic to them. (Same stuff you make signs out of.)
A wooden stake, right by the road, driven in a foot or so, with a 5LB hammer, tends to stay put. Show up very well in headlights. Makes it easy for the people putting out mile mark signs and water stops to find.
You can indicate on the yellow plastic something like M2 or W1 so they don't put water stops on mile marks, etc.
I put on a very official looking yellow safety jacket with reflective stripes when putting out marks. Keeps me safe and probably makes onlookers think I am from the city or something.
One night before a race, we were out looking for a mark while driving a car. Up comes a cop with lights on, he leans out and says "I am sure there is a good story you have as to why you are driving down my sidewalk in the middle of the night.... I am dying to hear it."
We step out wearing our 'official looking' reflective jackets and say "We are just attempting to locate a survey mark" and the problem solved.