When painting iron balusters, it's important to use the right type of paint. The paint should be specifically designed to adhere to metal. It should also provide a measure of rust prevention. Iron balusters must be fully protected from air or they will oxidize and begin to rust.
Metal paints are readily available in aerosol cans, making the job fairly easy. You should prime the iron before applying the paint. Look for a self-priming can of metal paint to reduce the number of coats you have to apply. Metal spray paint comes in a wide assortment of colors. Choose between matte, semi-gloss or gloss paint depending on how you want the balusters to look.