Assuming by "lawn" you mean a prepared bed in full sun, there are limitless options. As you may know planting perrenials directly into an established lawn is not a good idea since grass is very aggressive and will strangle your perrenials. I have a very large meadow area now that I prepared into an open lawn (cut out the edges, cardboarded and mulched for a period of time for the grass to die). There is now a nice selection of cool colored perrenials and ornamental grasses that bloom in succession and always provide me with something exciting to look at. For full sun, and well draining soil, I suggest these cool tone reliable perrenials: In the spring, I have white, pink and dark purple tulips (Queen of the Night is my favorite and long bloomer for me). They are immediately followed by purple alliums (I have Globemaster). As those finish, I have long blooming, intense blue-purple salvias (Caradonna, May Night, Blue Hill). Their time intercepts with the pale blue Walker's Low Catmint that reblooms everime I cut it. I wouldn't be without ornamental grasses mixed in with the perrenials for additional structure, texture and movement. By late June, my cool season ornamental grasses are in full swing - Karl Foerster grass gets frothy and tall swaying in the wind. By July, my favorite grass blooms among the perrenials - Molinia Ceraulea Skyracer. The grass itself is no taller/wider than 2-3 feet, but the airy plumes shoot up to 6-8 feet and create a transparent veil among the flowers that is pure magic. My verbena bonariensis blooms by now (annual in our zone, but overwinters for me in well draining soil, or will self-seed). I wouldn't be without its tall airy purple blooms that play very nice with other lower growing plants (and it blooms its head off until frost). At the same time (starting late June, early July) coneflowers are in full swing and last a long time too - I have Magnus, Pow Wow Wildberry and Pow Wow White, also love the more wild looking Echinacea Pallida. Another favorite perrenial (can be short-lived but nicely seeds around if happy) is gaura lindheimeri (I have white ones and pink ones). I love airy plants that weave into each other, bloom long and create a hazy, wildflower meadow effect among plants with more structure. In high summer, Russian Sage begins blooming cool blue for a long time (I recommend Little Spire since it's sturdier than the species). For fall, I recommend heleniums (I have the bright red Helenium Rubinzwerg). Other cool schemed plants I am in the process of growing, and have very high hopes for, include eryngium, althea cannabina, persicaria amplexicaulis, veronicastrum alba, and the red sanguisorba officinalis 'Red Thunder'. Finally, for areas with a bit less sun (dappled shade), I have found my favorite plant to be the ever-popular geranium Rozanne, which blooms intense blue-purple from end of June all the way into late fall.