1. Tap on the Frame icon at the lower left corner of the collage canvas
2. Select the background color icon on the upper left of the menu
3. Choose background
Please refer to the instruction collage below.
(2) tap on the "+" button at the center bottom of the collage canvas, and select "Change Background".
(3) Long tap on any blank spot on the canvas, and there will be a bar of backgrounds at the bottom of the page, you can choose any from them to set as the background of your collage.