As a longarmer I would tell you two things... if you want more design in the black aria then it should be quilted with green thread. That way the design will show up in the black and you will see what the quilt pattern is. However if you don't want more design then it should be quilted in the black. That way what ever shows (if at all) will only show in the green fabric and being that they are busy print, they wont show that much. There is my two cents worth.
Rose