That is a heavily discussed subject in the art world too. It's a little element of surprise. Much like the lone red poppy in a golden field of wheat. Many people feel that a monochromatic painting or quilt,(and a quilt is a work of art,)needs a little something to add interest. I'm back to the feeling that beauty is in the eyes of the beholder. Do it your way. If, after you finish, you think it is dull, appliqué a small, bright, flower which will blend with the room in which the quilt will be displayed. If the appliqué stinks, it can easily be picked off.