Screen changed color mode

I just made a call. I used my favorites menu to make it. First it ticked me off because the contact has several numbers and the software you use would not let me make the call without assigning that number to the contact. I don’t want my phone telling me what to do. I want to click on a contact and choose which number I want, when I want.
Then, when I take the phone down from my ear, the screen mode had changed from a white background to a black one and I hate it. please advise