Do not paint your porch white, it will get dingy looking in a couple of years here in Massachusetts. Rain, snow, sun, then rain again, ice melting and building up and melting again - it all takes its toll, especially since we are talking about horizontal surfaces.
Now, a house painted white is a different story. It stays looking good much longer than the porch...
Better thing to do is staining the porch with a dark stain of your choice. If you really prefer the paint, you can do so, but use a dark color as well. Dark paint is more forgiving when showing wear and tear, it also seems to be easier to repaint.
Here are pictures of a porch painted white, after 2 years in MA weather: