This looks like normal fall color (since lilacs are deciduous) combined with possibly an unconcerning minor amount of either scorch or leaf spot. Fungal and bacterial leaf infections can be quite common on lilacs, though this can be influenced by how wet the weather is at the beginning of the growing season. No intervention is needed right now, and just continue to monitor the plant for watering needs while it establishes. (Feel the soil about five to six inches deep and water well once it becomes somewhat dry to the touch at that depth.)

Miri