Spuria Iris?

Hollyhocks and Delphiniums?

You'll be hard-pressed to find something that blooms all summer, let alone almost all year long.

Why not ask on the Daylily forum for 4'+ yellows that rebloom (in your zone, the earlies might have three bloom periods). Of course, only the flowers will be that tall; the foliage will be much shorter. You'll have to mail-order in the fall, and most will be bare-root plants. They'll spend the winter growing a good root system and will bloom happily in the spring

I say yellows rather than yellows and blues, because daylilies don't really come in blue shades yet (lavender, yes; tiny blue patches, yes; entire flowers which are a true blue, definitely not).

Disclaimer: I'm not knowledgeable about daylily pests and diseases in your region, so you might want to ask about that also.

Here's the forum:
http://forums.gardenweb.com/forums/daylily/

Here are a couple of databases where you can usually find photos:
http://db.tinkersgardens.com/
http://www.daylilydatabase.org/index.asp

Ooops, obviously I thought you said blue rather than purple. There are plenty of purple daylilies, of all shades.

Sunstroke, that's my excuse -- sunstroke and the shock of finding two tomato hornworms munching my babies....

Don't know if your space ideas are realistic since taller things are also usually wider. If you can combine some lower growing items, then there are a large number of annuals that will provide bloon from May to frost. In your zone, a dwarf crape Myrtle will bloom for nigh on 3 months. Consider trellis or tuteurs and grow purple clematis. Coneflowers and rudbeckias are options. Knockout rose now has a yellow cultivar or consider other roses. For more purple there are some good options in foliage plants and shrubs. Drive around your town and see what's blooming.

Cephalaria gigantea - the plant that is the same height as the street sign and right beside it:

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Also Kirengoshoma koreana - lots of foliage per flower though.

Willow leaved sunflower might work better.

KarinL

Verbena bonariensis.

Actually the willow-leaved sunflower blooms pretty late, if I recall correctly.

There is actually a landscape design principle to be mentioned here. It is basically the key difference between landscape design and interior design, which is that in landscape design, the items you work with are not designed products whose attributes can be controlled (fabrics, art, furniture, rugs, paint, accessories, etc) but rather naturally-evolved life forms that are what they are, no more, no less. Kind of like you can't design a zoo saying I'd like some polar bears in small or some skunks in green and yellow.

Human intervention can give us odorless skunks, miniature poodles, and various designer plants like purple roses and so on. But with plants, still, you can only do so much, and so when you start with a vision and then fill in the plants, it can happen that the plants you "design" in your mind don't actually exist.

Now, because plants come in an already almost infinite variety, it is actually good design technique to design with just size and approximate shape of plant in mind and then fill in the actual plant name later. But you can really only do this when you more or less know what plants really exist and are hardy in your zone, and when you stick to fairly general terms. When it comes to determining more exact attributes, like colour and bloom time, it might create the kind of frustration the OP is currently experiencing to set your heart too much on certain details.

KarinL

How about cosmos? Yellow variety is cosmos sulphereus; regular varieties come in burgundy, lavender, and white. They're annuals here, but seed so freely I'll often get volunteers that survive our winters.

Depending on varieties and growing conditions (prefer lean soils so no fertilizer or much watering), they can reach 4' easy. The ferny foliage is pretty inobtrusive, but this simple plant when it covers itself in flowers makes a statement.

Trish, I answered the question sort of literally by giving some suggestions for plant cultivars that meet some of your spec's. From your post, though I suspect you need local garden designer to help you identify your vision and carry it out.

You could also use columnar forms of barberries with purple and yellow leaves - but they need to be taller than the front row plants from the start, for they really hate shade. (just rescued one from the shadow of a peony).

Thank you all so much for the suggestions. I am very grateful and will take them all into consideration

You could ask for suggestions on the perennials forum as well.

I'd agree that near year round bloom in a Louisiana zone 8 is not too likely, but for a zone 9/10 climate I'd recommend things like Wachendorfia thyrsiflora,Kniphofia uvaria, Dierama pulcherima, Chasmanthe floribunda v. ducktii, Neomarica caerulea, Salvias 'Indigo Spires', and Agapanthus 'Mood Indigo' or 'Storm Cloud'. Several of these can bloom for up to 6 months or more here in northern California, and all of them fit your height requirements.