I have a lilac bush that is about 4-5 years old. For the past 3 years, the flowers have been a light purple color. This year the flowers are white. The bush is healthy and growing fast. I've never seen a lilac change colors. Why did this happen? The color is not a problem, I'm just confused.
Optimizing the growing conditions and the health of the Lilac shrub will help in keeping bright, vibrant colored blooms.
Also dead heading the spent flowers will help.
It is also possible that you have a grafted Lilac. The bush may have reverted to the root stock, which is likely a different variety of Lilac.
Here are some articles with more information.