I love your fabrics, Sharon, and I agree that the all-dark flowers look better against the medium value of the background; that being the reason too, I believe, that the mixed do not have the same pop --- the medium value petals are getting lost against the background. Summarily: Value.
But the romance of the Civil War repros is there and, using each and every one of the fabrics in the collection, I would take this approach: Set all your flowers around the same-color centers as you have done. Mix the darks, lights, mediums on each flower and set them in place. Then, to set them up with high contrast, quilt with a decorative stitch on the outside edge of each "flower" in a very dark brown or even black - perhaps 30-weight thread (size 16 needle, topstitch [sharp], large hole).
Hope some of this helps.
~Jo