You can watch this video on how to do this in the studio. If you have a solid color this is done easily in the studio. If various shades you may have to use the pipette tool a few times.
For more complex backgrounds with various colors, you may want to use a imaging program. You can download Paint.net a free program, it is like an advanced version of Microsoft Paint.
Or you can use a web based one like Photopea or onlinepngtools, just search for it in google.