Testor Model Master Chevrolet Engine Red, a brilliant red-orange, is my favorite. Get the enamel, not the water-based acrylic. They make both kinds now.
1. Degrease the front sight with lacquer thinner, acetone, or something of the sort. Brush on several thin coats of the paint, letting each dry 24-48 hours before the next.
2. When the last coat is on and very dry, rub it with a pencil eraser. This will "flatten" the paint, eliminating all glare from it, while preserving the brilliant color.
3. When the paint gets dirty from shooting, repeat Step 2.
4. Keep all solvents and oils away from the paint when cleaning the gun.
5. When the edges of the paint start to wear away from the holster or whatever, strip it all off with lacquer thinner and start over. The paint will actually last a good long time if done right.
If there's no hobby shop in your town, check the fingernail polish colors at Walmart or a drugstore.