[Using "Select Sampled Colors"]

In my workflow, I often have to take an image that I've created in another program (a 3D program).  I set the background color there to be distinct from the object I want so that I can select it and make it transparent.  In AD, the process (the best one I could find) is the switch to Pixel, Rasterize, then Select -> Select Sampled Colors.  When I select the background color, the marquee does indeed seem to properly select the object.  Unfortunately, it doesn't really work this way.  If I cut (intending to cut out the background) color is lost from the object as well.  If I invert selection and cut (intending to cut the object for use elsewhere in AD) some of the background comes as if some of its color were still present.  I want the marquee to represent a clean separation between object and background so I can place my images with other layers in a final graphic file.  Essentially, I'm trying to treat "Select Sampled Colors" like magic wand.  Is there a way to do this?  The invert selection option seems to work better except that I still get a grayish, partially transparent box background instead of a fully transparent background.