Louise Beebe Wilder was an influential American garden writer who gardened in the south, and one of her books published in 1937 was The Garden in Color. She wrote:
"Purple is one of the most coordinating colors in the garden. It harmonizes all other hues and wipes out their differences of opinion, making them appear on friendlier terms. Unfortunately there are not many pure purple flowers - some Irises, some Campanulas, a few others but most going under the name have that wicked tinge of magenta which invariably makes for trouble rather than for peace."
It is true that not many perennials have that dark royal purple color, but there are some annuals-Verbena, pansies, Petunias and their smaller relative Millionbells-that can be used to edge or weave between perennials in a bed.
In the bulb category, there are dark purple tulips and hyacinths and some alliums with purple globes.
Callicorpa is a shrub with purple berries and some lilacs as well as the tall wildflower Iron Weed have rich purple flower heads.
Regardless of whether predominantly hot or cool colors are chosen for a garden, the color purple, as Miss Wilder said, can play a mediating role in creating color harmony.