I personally use medium to dark colors for backing most of the time. I've not had a problem so far with them, but I haven't used white or even really light pastels on the tops either.
Since you are concerned about the color, I would go with one of the lighter orange fabrics rather that the darkest or brightest orange.
You might also consider using a white polar fleece as your batting to eliminate see through. Although I think any white "felted" batting would work well for you. I would not recommend using a poly fiber batting as that would probably not be dense enough to prevent shadowing.
You've done a beautiful job on this!