If you like, you can change the background color in fullscreen mode.

To do this, press the menu button in the top-right corner of the window, select Preferences, and go to the Plugins tab. Then, check Fullscreen Background and click Preferences. Choose the color you want in fullscreen mode and close the dialog.

Now, when in fullscreen mode, the background color is the one you selected.

In order to enable this feature, you need to have installed eog-plugins.