I want to draw a box around something in my picture. When in Mark up mode, I choose the + on the very right side of the toolbar and it gives me the choices of the "Add Sticker, Description, Add Text, Add Signature, Add Shape". If you choose Add Shape you can choose from several shapes. After choosing your shape, you can use the tool bar at the bottom to change the shape "fill" color and opaqueness. That will show up as default as the gray circle and the "100" to the right of it representing 100% opaque, as in, you cannot see through the fill. The Shape outline color will be on the left of the bottom tool bar. The left circle by default is black color and "5" for line weight. Tapping on either of these allows you to adjust the Line weight or color, or the Fill color and opaqueness.
I did notice the Color menu allows you to add a custom color (chosen from the color grid) to the basic choices by tapping the "+" below the standard color option of Black, Blue, Green, Yellow, Red. It saves that color to your template. To get rid of the that custom color, long press on the custom color and then it gives you the option to delete that from the template.
Hope this is making sense. I didn't think it was working at first, but now I am seeing how to customize it. This does seem like more work to make a simple square, circle or arrow. By default they should not have any fill for the shape. They should start with 0 fill not 100 as most markups are trying to just highlight something within the shape, except perhaps the arrow and star.