Perhaps past prime bloom stage?

The one I got probably is, but it still looks good. I've seen these in several stores, such as grocery stores (which is where I got mine), and they're all the same shade. Really spectacular! I'd post a picture but don't know how to do that. They're feeding them something to get that color.

That coloring has nothing to do with fertilizer. It is just a matter of the specific cultivar of hydrangea and how the blooms age. With most bigleaf hydrangeas, the color of the aged flower is distinctly different from that of the new flower - a maroon or brownish tinted flower is very common on those that initially bloom a deep pink, like 'Glowing Embers'. And if not a bigleaf hydrangea (H. macrophylla), nearly all of the paniculatas age to a rosey, deep pink that can deepen to a maroon, wine or brownish tone. The amount of sunlight they receive as well as moisture as the flowers age has as much bearing on the coloration as anything.

Without seeing any photos of the flower in question to confirm type, etc., it is also possible that the coloring is the result of dyes. Placing cut flower stems in colored water will affect the flower color - florists do this all the time.

It's H. macrophylla. I went to the store and took pictures today. Got a pic of the info on the tag, too. It's a deliberately aged flower, but it still looks really great, and it also looks like the stores are selling a lot of them. I've never seen anything like it and was awed at the row of them inside the building when I first saw them.

Here's a link to the producer's website:
http://www.hanabayflowers.com/careculture/HydrangeaVintageHarvest.pdf

My only experience with hydrangeas was an oakleaf, years ago, and it lived about a year. I don't know what went wrong, and don't know how well they do in Houston's climate, but am going to plant this one. There are several pieces in one pot and I may divide them and plant part in the ground and part in a pot, in a shady area so they'll get morning sun, and see how they do.

Here is a link that might be useful: {{gwi:1024914}}

The producer's (!) website says: "We�ve heard that some
gardeners mix in rusty nails in the soil but we don�t know whether that helps or not."

The answer is NOT.