To me, color all season means either annual (start to bloom early summer and go till frost, some reseed each year) or choose a few different perennials, because most of them have a limited couple weeks of bloom.
I also like my iris best if I divide every few years and keep just the best parts, allowing room for other plants. I don't like a full bed of iris alone in summer, when it's done bloom and just a lot of leaves.
