Color - I just paint them black but really any color will work fine. Black tends to give up heat a little faster than some other colors but the temperature difference is not that great.
Type - I like a semi-gloss or a "satin" finish. You can use a flat paint but I don't think that the surface of a flat paint sets up as hard and is not as durable as a semi-gloss or satin finish. You can also use a glossy finish but beware of the fact that a glossy finish will tend to show any mistakes in your paint job (runs, drips etc.). A glossy finish may also appear too shiny and show reflections.
Paint - My favorite transformer paints are the Krylon Fusion paints. Yes - These are specially formulated for painting plastic but they also work well on metal surfaces. The satin black is Krylon's #2421. It sets up with a hard and durable top surface that can be cleaned. See the link below for the different colors that are available. Any color beginning with a 23 is a gloss paint. Any color beginning with a 24 is a satin paint.
Krylon Fusion paint
Paints that I do not like are the high temperature "barbecue" paints. The one made by Krylon #1618 is not as durable as the Krylon Fusion #2421. Rustoleum also make a high temperature paint #249310. I don't like this one because the color (to my eyes) is a very dark gray rather than a true black.