Also known as Sweet Rocket, Dame's Violet is a pretty biennial, bearing white or purple flowers similar to honesty. A packet of seeds is likely to produce flowers varying in colour from deep rich purple, through all the paler lilac shades to white – leave them to seed themselves and they will maintain this diversity indefinitely, gradually naturalising over a large area.
Sow seed in late-spring where you want it to flower. Deadhead spent blooms to prolong flowering, but allow some flowers to develop seeds so plants can self-seed around the garden.