They soften the hard planes of a house. They help make a house a home. Geraniums are the Germanic choice, but there are so many more plants suitable for window boxes. You can pick a favorite or go with a mixed planting that resembles an untamed English cottage garden. Colors can be coordinated with a house.
Soft pink impatiens or petunias flatter stern gray-stone walls.
Some garden designers hold that grouping one flower color makes more of a visual impact than mixing flower colors. But window-box builders should be allowed to please themselves. Flowers never really clash with each other or anything else. One very popular color scheme now is inspired by the Monet gardens of Provence. That sunny yellow and assertive blue are everywhere in home decorating, in fabrics, paint colors, wallpaper, and gardens, too.