Sketchup models very dull

I made a whole city using Google Sketchup and imported the fbx model of the same. In sketchup the city and the textures look vibrant and all. But in Unity they are really dark and dull. Half of the time they just appear as grey buildings!!! Can someone please suggest what is the problem? I also have a directional light in the scene. Everything else except the sketchup models look really as they should, but not the sketchup models. Can someone please help me and tell what the problem is???(Along with the solution...)My city is looking very dull because of this...