They used the perennial candytuft (Iberis sempervirens) which is white and has evergreen foliage, with annual blue lobelia (Lobelia erinus) and annual red geranium (Pelargonium peltatum) in the center. I think they used the candytuft around the geraniums, so it was planted in the order of "red, white and blue". I can't swear that candytuft blooms all summer, but I've read it blooms at least through July and reblooms in fall if deadheaded, and likes good drainage. It might be worth a try.

I have used white geraniums in hanging baskets and containers, and have been disappointed by the ratty way they look when the blooms fade. I like white star flower (Pentas lanceolata) better, no deadheading and they can really take the heat.

I used annuals, I don't think hardy geraniums come in red, just white and shades of pink and purple-blue.