When you get ready to construct the house, spread your icing directly on a strong piece of cardboard, plywood, or a flat unbreakable tray to cover area where house will be built. This helps to create a strong foundation. To prevent having a lopsided house, be sure to hold the cookie sides in place until dry, or they won't line up. My kids had trouble with this step because they were so excited to decorate the house with candy. You don't need a large quantity of candy to decorate.
Variety is the key and less is truly more.
Food coloring can be added to brighten up your gingerbread house. However, certain food coloring will thin out your mortar and in most cases the candy will provide enough color for your house.