Irradiate the sugar-salt mixture for a week or two with a 200-Curie Cs-137
source, or something comparable. The salt turns a deep yellow color while
the sugar turns brown. I don't know if you can then distinguish the two
colors for individual particles, but that's your new problem.
After separation, heat the salt on a hot plate to return back to its natural
color. I wouldn't recommend doing that with the sugar.
If it's iodized salt, the color is more gray.
Tom Walkiewicz
Dept. Physics
Edinboro University
Edinboro, PA 16444