Answer: Generally speaking, winter color is provided by evergreens along with berried and fruited plants and sometimes too by the color and texture of branches and bark. You might look into something like crabapple trees (some have white blooms such as Malus x "Sugar Tyme", Aronia arbutifolia and Ilex verticillata -- both of these have small white flowers and excellent berry displays, the twiggy shrub dogwoods with colored bark (creamy white flowers), and evergreens such as the many assorted junipers and mugo pine, for example.
Winter flowers are harder to come by but you might enjoy the vernal witchhazel, Hamamelis vernalis, a native shrub with yellow blooms during sunny spells in the winter. The evergreen Pieris japonica shows its flower buds all winter and blooms very early. Earliest spring bloomers would include Chaenomeles shrubs and the Hellebores which are perennials for shady spots. Sometimes pansies and violas planted in the fall will survive the winter and bloom during mild spells, then rebloom in the spring.
For summer blooming shrubs in white, you might like the hardy hibiscus shrub sometimes called rose of Sharon (Hibiscus syriacus), there are some hybrids with white flowers and they bloom during the mid to late summer. Spirea nipponica "Norwegian Snow" and the small Deutzia gracilis "Nikko" are nice smaller shrubs with white flowers in the spring. Mock orange is an old fashioned favorite with fragrant white blooms. Hydrangea paniculata offers late summer blooms in white on a very hardy shrub, while the equally hardy summer blooming Hydrangea arborescens also blooms in white. You might also like sone of the hardy landscape roses or Rosa rugosa cultivars with white flowers.
If you need a reference source for information about these plants you might like the Kemper Center web site, it is based at the Missouri Botanical Garden.
I hope this gives you some ideas.
Q&A Library Searching Tips