In mobile prototypes, the background color is dependent on the content in the screen's image.
InVision pulls the four corner pixel colors and tries to match the background as closely as possible. If you've set a background color in your design, that color should remain as the background color.
However, please note, if you export an artboard with a transparent background, the transparency can result in a number of different background colors being set on the mobile screen as the system hasn't been told what color to actually use. We advise you to always set a background color on the artboard if you're working on a mobile prototype.