You know the old blue photo paper you would lay in the sun with a leaf (or some such thing) on it and the sun would work with the paper to make a “print” of the leaf? My Hydrangea is doing this same thing in miniature.
The florets (which start out a bright periwinkle blue) have flipped completely over so what was the bottom is now the top. And they’ve turned this deep vibrant pink. If you flip the floret back over (I’m holding the one on the left, below) it’s almost completely white on the underside, which is what used to be the top (blue).