I would use darker colors like black, brown and red. Lighter color tend to show more imperfections. Exterior doors take a lot of abuse, so darker colors will hide the scuff marks and dings that front door will inevitably have.
As a general rule people go with subtle colors for the outside doors of a house. I have seen plenty of houses with high contrasting colors that look ridiculous, like an egg shell colored house with red doors and red trimmed windows. A neutral color like "off-white" or cream will look great with just about any color house.
The paint choices for exterior doors is limitless. I would suggest painting the exterior doors in an accent color. You should choose an accent color that is within the paint chip family of the main color of the house. However, the choice is completely individual.
Painting any item for a house, whether it is a door or a wall, needs to take into consideration the other colors that are around that item. For example, many people love a dark red door with yellow siding, or a black with green. Others would like a light color, like sky blue, on their door because it is inviting.