The 60/30/10 rule is a good rule of thumb: 60% of a main color, usually the siding or body color; 30% of a second color, usually trim and 10% of an accent, often found in front doors, shutters, garage doors, brackets, etc.
Front doors can often relate to garage doors, if the two are located close to each other. They can also connect to shutters or other elements, again depending on the balance of colors, location, etc. Most often, front doors are their own unique color and a great place to paint your favorite color!