Purchased a limelight hydranga about six weeks ago. Was told to put Miracle-Gro on them.
Limelight hydrangea flowers go from a brilliant white, to green, to a peachy colored brown under normal circumstances so I'm not sure if the plant food has changed the color or if this is part of the normal fall color. They generally don't need plant food. If you followed the directions on the Miracle-Gro they should be ok. They will not go back to the original color in any case however they should produce bright whitish green flowers for you next spring.