I just picked up a blue shasta daisy from Spring Hill's catalog that my husband wanted badly enough to help be dig a new bed...

maybe white hollyhocks beside the hydrangeas?

and a blue hosta. maybe one with the 'seersucker' texture to the leaves. they can take as much if not more sun than the hydrangeas.

I'm a fan of spiderworts- the blue/violet color makes me drool every year, but some people think they're weedy.

You didn't say if you wanted to stick with blue, white, and yellow, but I assume that is so. Here are the flowers that have worked well for me. (The links are to the Hortiplex pages on these flowers.)

I actually prefer
Japanese aster (also called Oriental boltonia) to Shasta daisies because they don't flop, they don't need to be deadheaded, and once they start blooming June, they don't quit. The flowers are smaller, but they really fill up their space well.

Love-in-a-mist (Nigella damascena) is a nice little self-seeding annual with frothy foliage and blue flowers. It grows about 1' tall for me. It blooms in May. I let it sprout wherever it wants since it makes such a nice little filler.

Happy Returns is a wonderful little yellow ever-blooming daylily. It has a nicer lemon color than that Stella D'oro you see everywhere.

Last, I must put in a plug for blue mist shrub, Caryopteris x clandonensis. Its silver foliage is a nice contrast and it blooms a beautiful hazy blue in the fall. It blooms on new wood, so you can cut it way back early each spring to keep it to 3' tall and wide.

Happy gardening!

You did not mention whether this is a sunny site, but I shall assume that it is. I would also consider Homestead verbena as a flowering groundcover which is lovely and it spreads but is controllable. My white daisy is "Becky" and remains very neat. I do like yellow combinations with purple, lavender, blue and also silver such as artemsia, dusty miller, lambsear, and of course, chartreuse with the purple is so pretty, i.e. lady's mantle(alchemilla mollis).

Hope that helps
Vicki