I am partial to a red door. Although it does run the risk of looking overly Christmas-y, it is also warm and inviting. A more conservative choice would be to paint the door green to match the roof– it would create a nice feeling of balance, especially if you dress it up with some nice hardware.

I think a white sided house with a green roof would call for a red door for a bold accent or for a more muted effect, a matching green door. If you are looking for a more dramatic entry way, a solid wood door with glass is always classic too.