What is the rule file and start rule assigned to a building? Does this rule exist in your workspace?
Click on a building. In the Inspector, is the attribute Representation set to "realistic with facade textures"? If it is set to "solid color", which is the default, then the buildings will not have a texture from an image but will rather be a single color.
What are your viewport settings? Are textures enabled (shortcut keys 6 or 7)?