Question
why is my blue hydrangea turning a green color

Answer
Hello Marilyn

Sometimes a blue hydrangea will go green in a very warm climate, but there is no definite reason why it happens.  The hydrangea may bloom green for a year or two and then go back to being blue!  

If the color change really bothers you, you may be able to help matters by watering-in a coloring agent, which you can get at the garden center.

Also bear in mind that as the season progresses, the blooms will fade to a light green before turning brown in the fall.

Gill