Outgoing call background color

When I make an outgoing call on my iPhone, sometimes the background is gray and sometimes black. It’s has nothing to do with the call being made to an android or iPhone. This screen is black for some calls I make to android phones and some gray. Same with outgoing calls to iPhones. Can someone please explain why the background screen switches between black and gray? And it doesn’t switch colors during a call. It’s either black or gray for the duration of the entire call.