Is there a command to remove the desktop wallpaper and use a solid color?
An often described way is to use
reg add "HKEY_CURRENT_USER\Control Panel\Desktop" /v Wallpaper /t REG_SZ /d "" /f RUNDLL32.EXE user32.dll,UpdatePerUserSystemParameters
However, this doesn't work on Windows 7 (64bit?!) anymore; see this Technet post.
A proposed solution is to use the SetWallpaper tool. To remove the wallpaper run it in an elevated shell: