The best thing to do when choosing the correct background color is to use the Painting Preview. On the main UI, load your image and then choose the white background color. Now click the Paint button and choose Painting Preview. If the colors look washed out then try changing it to a black background color and view the Painting Preview again. Every image is different so it's hard to say what background color to use. The tutorial recommends white because that is the color that works best with most images.