Hi Lars,
If you apply a colorpict using a grayscale image, you only get gray modifications to your material color. If that material has color, then you will get a brightness modification to the color specified in the material primitive. I don't know of any reliable procedures for taking color information in a subset of an image and extrapolating it into the surrounding image. If your color image only covers a fraction of your surface, you are not going to be able to get color for the rest of it by combining this somehow with a complete grayscale image.
-Greg