background color hashing?

[Jeff Dege]

I have a mapfile.  

In the map section, I have:

   IMAGETYPE GIF
   IMAGECOLOR 191 191 255

In the first layer, I have:

   LAYER
      TYPE POLYGON
      CLASS
         STYLE
            COLOR 255 255 255
            OUTLINECOLOR 0 0 0
         END
      END
   END

I use shp2img, to generate a GIF.

On the GIF, the areas not covered by my polygons are colored the light
blue that I want.  But the areas that are covered by my polygons aren't
white, they're a blue-and-white check. 

For the life of me, I can't see what might be causing this.