I would say shasta daisies. White blends with almost everything and they bloom a long time. White Swan echinacea would be another choice. If you're not growing from seed, they should be available in early July in most garden centers. If you are growing from seed, they take at least 2 years to bloom.

Coreopsis is a favorite of mine.

Janie

I think something very light, maybe white. There are lots of possibilities, shasta is a good one, daylilies, more iris in white or very light shades, feverfew, centranthus, maybe a lemon yellow to pick up the shades in Kate and the hosta.

It looks pretty nice as is!

Shasta Daisy "White Knight" is a First-year-flowering one.

What about some of those lilies you mentioned elsewhere, along with the Shastas?

Nell

It doesn't look like there is a lot of room in that spot (you are puting the plant in front of the Baptisia and beside the Tradescantia right?), so I would suggest something compact. I like the idea of a white plant there since you want to compliment the things around it.

How about a Peach Leaf Bellflower, Campanula persicifolia? I would go with a white one, but there are blue ones and a couple of double cultivars. Mine get near 30 inches tall. One of my favorites.
Astrantia? Amazing flowers. It comes in whites, reds, and pink shades. It would add a nice texture too, since the leaves are kind of jagged.

It doesn't meet your height requirements, but Geranium 'Vision Violet' would look nice crawling at the feet of the Baptisia. Mine blooms a lot longer than my other geraniums and it can bloom again in the fall.
CMK

Here is a link that might be useful: Campanula

Casablanca lilies. Or moonbeam coreopsis. Light, airy and comes in light yellow to pink shades.

My baptisia grew to 3 feet wide within 3 years. So, I wonder about space for perennials. I think I'd go with an annual.

White Angelonia -- blooms all summer. I grew the lavender last year and just loved it. About 24" high.

Diamond Frost Euphorbia -- I've heard a lot of good things about it and want to try it.

Cameron

What about dusty miller? In my zone it can overwinter some times but probably won't in yours. It has nice greyish white foliage, yellow flowers when it blooms, and you can buy it at a big box store for next to nothing.

If you like dusty miller, but want a perennial, go for one of the armetesias. They're called wormwood and have a slight odor. I wouldn't say it's entirely pleasant, but it seems to affect people differently. It's also said to keep some critters away.

Be careful of the artemesias, some are "thugs". I have artemisia "Oriental Limelight" which is very pretty but could spread to timbuck2 if I let it. Every year I yard out some to keep it under control. If wormwood is the same, which I seem to recall it is, you may want to research first. There are other var. tho which might be OK.

I've even had dusty miller overwinter here in zone 3 altho they are never as pretty so I've quit trying to keep them over. Might be OK in your climate.

I like baby's breath with roses, very old fashioned, I know.

Not white but I like chives for the shape and they are a good companion plant for natural insect control. A mature clump is not quite 20" but probably 12"- 14".

Lamb's ears have a lovely soft texture and silvery leaves. You could cut off the flower if you didn't want purple.

Lady's Mantle is nice with roses and would repeat the lime you have there now.

Something taller(2ft) and yellow?
-B

What about Tall Garden Phlox(Phlox paniculata)? They have a long bloom time, variety of color choices and are fragrant (especially the white). Go for a mildew (powdery) resistant variety though.

Janelle

I also like the idea of a threadleaf coreopsis. Also one of my favorites, particularly as a filler. I would try and find one that isn't yellow (I seem to recall these existing...I know there's at least one with a contrasting red eye), considering the yellow foliage plants growing below.

Janelle

I have not grown this, but found it on the High Country Gardens website:

http://www.highcountrygardens.com/catalog/product/77627/

Physostegia virginiana 'Miss Manners'

It looks and sounds, from the description, as if it could be good for the spot. I have ordered from this company many times, and can recommend the company. I was thinking of the David Phlox(I have this plant), but it is probably too tall for the spot.

Here is a link that might be useful: High Country Gardens Miss Manners

Oooooh, what about Liatris? There is the vivid purple or the white...

Janelle

You have a beautiful patch of flowers there! Nice cerise, the deep blue of the baptisia and the chartreuse of the hosta. How about something dark like a chocolate cosmos? It would have the effect of making the colors you are already using even richer.

I think I have narrowed it down to the following candidates:
1) Baby Swan Ecinacea (white, shorter than reg. White Swan) or Merengue (sp?) echinacea
2) Coreopsis Snowberry
3) Verbena Snow Flurry

These items should be very long bloomers in my garden, so will look good with the rose and spiderwort once the false indigo has finished blooming.

The very first thought I had was white or yellow.

Libby

I'm a huge fan of snapdragons for filler plants. You can choose just about any colour or size. They come in dwarf, medium and tall; they re-seed, are great cutting flowers and don't mind a bit of shade. If you can find them, the Madame Butterly doubles are just gorgeous (I grow mine from seed; it's the only source I can find for them). I agree with Libby: bright yellow planted maybe with cream.