You need a color that won't clash or blend in to the point that you can't tell which is which. I would go with a midnight blue. It's very pretty, reasonably subtle and will go with the other colors rather well. It'll have a touch of elegance to it.

I think you should paint the door red. It would go with your reddish roof and your taupe house. Plus, red is considered good luck and makes a bold statement and enhances the overall appearance of your house.