Is there something that I can do to make it white again? Thank you so much!
You are probably referring to the change in color that many hydrangea blooms have, starting at the end of summer (more or less). If so, the answer is no. Hydrangea blooms naturally go through a color change that varies dependng on the variety. Some start white then they turn a shade of pink and finally turn brown. I have some that start pink, turn dirty white, then green and then brown. You can deadhead them if you do not like them or enjoy the plethora of color changes that they grow through.
For information on deadheading, click on the link below and scroll down to the paragraph titled Removing Old Blooms.
Here is a link that might be useful: Deadheading and Pruning Info
In other words, the "greening" is a normal process of aging.
Next year, the fresh blooms will again be white.