It either is a more vigorous sport (genetic mutation in a branch) that is more vigorous and so has outgrown the white parent or the purple ones have suckered (sent out underground stems to sprout a bit farther out) that are more vigorous bloomers than the original plant or there were two cuttings (purple and white) in the original pot these were planted from and the purple one has taken over. I haven’t ever had lilacs seed so I don’t think it is that.

(or whomever mows the lawn ran over the white one and replaced It with a purple one by accident.) 😏

These are large old bushes. The 2 coming in one pot sound possible. At one time a serious gardener lived here. You gave us a laugh with your last comment. Thank you for responding.

I just read that some lilacs are grafted. So that could be the answer. The serious gardener might have bought a grafted plant.