Fit Canvas to UV
When you move UV polygons in the Texture view, you are free to place them outside the texture. Exactly what is then mapped to the associated polygons depends on the object’s Texture tag. If Tile is enabled, the texture will be tiled. Otherwise, a neutral gray color will be mapped to the polygons.
But if you are like most people, chances are you have moved UV polygons outside the texture because you want to increase the size of the texture. In such cases, choose Mesh / Conversion / Fit Canvas To UV. The texture will be resized to fit the UV and the new pixels will be filled with the background color. See Tweaking the UV Mesh for an example of this technique.