Setting the "outside" voxels only to a different color is a more complicated process because it is not done in the AFNI image viewer.

If you are creating images for publication (or PowerPoint-illization, etc.), then the simplest thing would be to import the images into an image editor such as gimp or Photoshop and flood fill the outside with white.

If you really want to do this in AFNI, then you can do the same thing (flood fill) in the Draw Dataset (AKA Draw ROI) plugin. If you have never used this plugin before, then I could describe the series of step you'd use. It's a little time consuming, so I won't post those here unless you really want. AFNI isn't really a very good image editor, and that's why I recommend using an external editing program to make the images look snazzier for presentations.