If you're using a new.1 handset with Back, Start and Search "soft buttons," you can configure how they appear in the software-based navigation bar onscreen. This navigation bar can be colored to match the background or accent color, or can always be black to match the handset's front panel.
The ability to use software-based Back, Start and Search buttons is new to Windows Phone 8.1, and gives device makers some flexibility when designing new handsets. One of the design considerations was that it lets them develop a single phone platform that can work with both Windows Phone and Android, which also supports soft buttons.
I'm only aware of a few low-end Windows Phone 8.1 handsets that use these soft buttons, such as the Nokia Lumia 630 and 635, the latter of which I'm currently evaluating. On such devices, the screen has a non-standard 854 x 480 resolution, rather than the normal 800 x 480 resolution. The 54 pixels of additional space is used up by the navigation bar, which looks like so:
I was interested to see that this feature was customizable. (I guess I sort of assumed it would always have a black background to match the device face.) But it is: Navigate to Settings, System, Navigation Bar to view the new Navigation Bar settings interface.
Here, you can choose between the following three choices:
Always dark. In this case, the background of the navigation bar will always be dark (black).
Match background. Here, the background of the navigation bar will match your theme's background color (which is set in Settings, System, Start + Theme). If you use a dark background, the nav bar's background will be black. If you use a light background, the nav bar's background will be light gray (and not white as you might expect).
Match accent color. With this option, the nav bar background will match your theme's accent color (also set in Settings, System, Start + Theme). Here, you can see how it looks in cyan.
Fun fact: The navigation bar doesn't show up in screenshots. Which I guess makes sense.