When it comes to shoes, the best time to buy them is right after you find your dress, and well before you have any alterations done. So when you find the right shoes with your dress, it will be a lot easier to determine the hem length, thus tailoring to your overall polished look.
When it comes to color choices, the hue should either match perfectly with the dress or be a complete contrast of the color spectrum. If you love the shoe, and the color isn't a match, there is the option to dye.
Another helpful hint about shoes in general is that if you are not accustomed to them, wear them inside the house only. This helps allow for muscle memory and to gently break them in to mold better with your feet.