Posted: 16/07/2014 at 22:24
Usually they stay one colour. As Dave says, pull off the purple bit as low as you can where it joins the main stem. Take cuttings off the strong white part.
A lot of variegated plants will throw up a plain green bit. The plain green bit is stronger than the variegated bit and will take over the plant if left.
My white flowered buddleja has always stayed white. Usually I have trouble with the weak variegated leaf ones reverting to green leaves.