My lilac bush is around 12 years old. It is a purple old-fashioned variety. This year, instead of purple blooms it is almost white with purple tips. What does that mean? Also, should I be fertilizing it and, if so, with what?
Lilac Bush Color
To answer your second question first, lilacs don't need much fertilizer. This article will tell you more about lilacs: https://www.gardeningknowhow.com/ornamental/shrubs/lilac/lilac-care-growing-and-planting-lilac-bush-plants.htm
Regarding changing color, it's not unknown for the color to fade because of incorrect pH. Lilacs prefer soil that is slightly alkaline. A light application of lime once a year should help, but you might want to get a soil test just to make sure.