No, there is no command to switch the background color. This is a known problem where the display is affected. If the Fax is printed out, it will print correctly, however, sometimes, it prints in inverse video. This problem also occurs on Power Macintosh computers.
To resolve this problem you should upgrade to Express Modem 1.5.5 software available from the Apple Software Updates Web site at
http://www.apple.com/swupdates/ .