Many people use model airplane and car paint.
This is available in MANY colors, and in flat or gloss.
It's available in solvent-based, or water-based.
Since it comes in very small bottles and is fairly cheap, you don't waste it like you do with larger containers that dry out and go bad.
It's easy to apply, drys fast, and is easy to remove.
Always STIR thoroughly, since it settles.
You can experiment with different colors AND with flat or gloss finishes.
Some favorites for front sights are:
Gloss White.
Red-Orange.
Fluorescent Orange.
Yellow.
Bright "Safety" Green, which is actually the most visible color in daylight.