I have this game with a salmon background color. I can set the background to the proper color into the create function with:
game.stage.backgroundColor = '#f3cca3';
The problem is that it takes a little while preloading the images and sounds and so it stays black in the meantime. I tried:
var game = new Phaser.Game(window.innerWidth, window.innerHeight, Phaser.CANVAS, 'meems_house', { preload: preload, create: create });
game.stage.backgroundColor = '#f3cca3';