The BEST I have found is Testor's model paint in the 1 ounce bottles. Sold in hobby shops like Michael's for about $2.50 and comes in about any color you can desire!

Clean all oil and solvents from front sight with Alcohol just prior to application and don't touch with your fingers! Use a wooden Toothpick to apply 2 lighter coats rather than one heavy one. Some apply white as an undercoat then orange, red, yellow, green etc. as a top coat, but I have never found the need to. Allow 24 hours between coats and trim excess after it dries with a new razor blade.

Lasts for years!!