Blanket Flowers (Gaillardia) are an excellent flower for all season colour. They drop seed too so, while they aren't a long lived perennial, there are always new plants coming up. They have some nice newer varieties of it as well.
http://www.perennials.com/seeplant.html?item=1.220.080
I can see some Speedwell (Veronica) at the back with the Blanket Flowers in front. Or you can do Daylily. There are so many wonderful varieties and colours of them you could fill in that whole area quite nicely and not have to worry about them for years. They drop seed as well if allowed to develop seedheads. They just don't flower all the time but they do for quite a while.
You could try some anchor plants at the ends that are a little taller. I mentioned the Yucca earlier in the thread. They would be neat at either end. When they flowered the blooms would reach up on both sides of your window and be quite striking.
http://www.youcanlearnseries.com/Lan...obsLadder.aspx
What colours are your favourites? That may be a good starting point.