You know those pastel-colored dry-erase markers that are two steps from useless when you're trying to communicate information on a whiteboard? Lime green and so forth.
If you have a white-on-black fixed-sight setup (bar dots, three dot), fill in the white areas on the rear sight. They're still visible, but since the value is distinctly lower, your eye has a much stronger affinity for the front sight. And if you don't like it, you can take it off with a Q-Tip.