Lilac tree was once purple and now is white.

I saw a discussion here about this. I can not find it however. I know I am not crazy because I just found the proof I was looking for. For the past couple of years I have said to my husband that I swear that my white lilac was once purple. I went back into my pictures and found the proof. I have lived at this house for ten years now and my lilac changed colors. In 2008 it was blooming purple. There was a lot of old wood growing. I decided I wanted to cut some back. I did some research that said to cut one third of the plant at a time. I did that every fall. In 2018 my Lilac was looking thin and sad so I decided I decided to start from the bass of the plant. I cut it down and allowed new growth to come in. That is when I got white lilacs. Did I shock it? Did I cut the purple grafting off? Why did it change? Will the purple come back as it ages?