Scuff-sand the surface to remove gloss and create a micro-jagged surface (aka "tooth") the new paint can engage to.
For exterior doors my first choice is a marine LPU, but unless you paint boats (know anyone?) the cost of hardener, reducer, and other kit is prohibitive.
So I recommend a quality alkyd enamel ("oil paint"). Brush it unless you are setup to spray, get throwaway bristle brushes. Enough coats so the old color doesn't punch through.
Start with a wipedown with the paint's solvent to remove dust and surface contamination.
To buy the paint, either use a Rustoleum stock color (beware, some Rustoleum is latex), or go to a Sherwin Williams industrial outlet who can custom match and give you quarts. A more hooty-tooty brand is Fine Paints Of Europe.