When an EPSF object is selected, you can't select the Image Colors command from the Special menu in the Color Palette.
MacDraw Pro cannot extract the colors from an EPSF image. However, you can use the eyedropper tool to pick up colors from the PICT Preview part of the image.
The colors that you see are from the PICT preview portion of the EPSF object. These PICT colors are merely approximations of the actual colors, which are in the Postscript part of the EPSF object. Even if Image Colors were selectable for EPSF objects, the extracted colors would be of little use since they are not the exact colors used for printing.