1. Open the photo, click on the picture, enter the page, click on the upper right corner of the Edit. 2. Click on the mark icon to enter the page, click on the lower right corner of the plus icon, click on a mosaic icon styles, to the box as an example, click on the small box. 3. Then hold down the black icon and select the solid fill color. 4. You can see the fill color, click on the upper right corner of the Done, and then click on the lower right corner of the Done.