Annuals that deliver good white blooms include petunia, wax begonia, nicotiana, alyssum, and impatiens. Green is a good compliment to white because it helps your eyes recover from the strain of bright white. Don't forget the shades of green in leaves as part of your color plan. Variegated or multi-toned leaves add a splash of interest, but extensive use tires the eyes. Just don't over do it.
Mixed colors add a festive touch.
But, too much mixing can be more disorganized than festive. Repeating three to four colors different places in the garden or flowerbed helps tie everything together. Create transition zones between bright colors with greens and whites. Progress from color to color using intermediate tones. For example, shift from reds to pinks to blues across a bed.