Smooth Hydrangea (Hydrangea arborescens, the species this cultivar is derived from), does not produce fresh flowers all season long; their bloom window is typically concluded, or nearly so, by now. Flowers often age to green before finally drying to pale brown.

Soil pH is not a factor in blossom color in this case, as it only impacts Bigleaf Hydrangea (Hydrangea macrophylla) and its cousin Mountain Hydrangea (Hydranagea serrata). Even for those species, pH will only alter the color of fresh blooms, as these too fade to green regardless of their original color. pH plays a role in how the pigment in the flowers is able to make use of aluminum ions absorbed from the soil, but Smooth Hydrangea does not make these pigments since flowers are white.

That said, all of these Hydrangeas do prefer acidic soil to best absorb nutrients and grow well. Unless a soil test shows a very low pH (not likely, as it's hard to achieve and wouldn't likely be naturally occurring in the suburbs), the use of lime is not needed, nor recommended as misuse if pH is already near-neutral could result in nutrient deficiencies.

Dead-heading might allow for a little bit more rebloom, but since Smooth Hydrangea doesn't keep producing new flower buds season-long, there is limited capacity for it to be impactful. It won't hurt, though, and may at least provide aesthetic benefits if it helps keep stems upright and not bending under the weight of the spent flower heads when they become laden with dew or raindrops.

Miri