There is a lot of purple in May and early June. Here are four purple-flowered plants, two natives and two not, that have done very well. They will all take light shade, although the geranium would probably be happiest with more sun than the others.
False Indigo, Baptisia australis
This native plant erupts from the ground the second week of May, and is blooming by the end of the month! It is very popular with our bumblebees.