We all have such different opinions you know you're going to get 50+ different ideas. :D And in the end you'll still have to make that final decision! :shock: For what it's worth, and it's not much...
I'd use that light cream (is it the yellow you mentioned?) then the third color, that deep red and then the flowers again if you have some left. I don't know if I'd make it 7 inches wide but at least 4" - 5". What ever you do it's going to be beautiful!
Of course you could run all 5 rails around the whole thing.
Just keep putting them up against the quilt. Pretty soon something will say, "It's me! I'm the one!!!" and you'll know it when it happens.