Is it possible to create (using grafting) a Lilac (Syringa vulgaris) which has different colors? One branch with white flowers, the other with lilac?
It would appear you can:
This page is specific for lilac trees and includes some grafting advice. In particular, quote:
Functions of Grafting
Grafting brings new cultivars to an existing tree to introduce another variety while a cutting can be grafted to a new seedling to bring strength to the new planting. Planting a grafted lilac branch can lead to a new bush altogether.
The list of expert answers has a number of references to lilacs reverting to their rooting stock colour: (ie. multiple colours are possible on the same plant via grafting)