Hello, alemania. I cannot speak for all the occurrences of purple leaves that you mentioned since there is only one picture so, these comments only apply to that one instance. But hopefully it applies to them all.
What you are displaying in the picture is normal leaf colors that can occur at the end of the growing season -when chlorophyll production tapers off- so good luck to you, no phosphorus issues there.
Hydrangea foliage can turn various colors in the late Summer and early Fall. It is a function of your geographical location, how strong your sun is, minerals in the soil, the type of hydrangea, amount of pigments that have developed in the leaf, etc.
That head turning type of purple foliage effect is a superb example of what you can see with most H. quercifolias (aka, oakleaf hydrangeas) and a few others. And this is not limited to oakleafs; some mopheads and lacecaps can also put nice displays as well.... but not many.
Because there are many variables needed to produce a good show, you do not necessarily see the same exact Fall color foliage every year and on every one of that shrub. So shrub #1 may do it only on some years. Nearby shrub #2 may do it in different years. One planted on the other side of your town may not display foliage like this. And so forth.
Only oakleaf hydrangeas and a few others will almost reliably put a good show in the Fall.... provided it is not a cloudy/rainy Fall. By the way, I should note that if you get cloudy weather, for long enough time, you may actually trigger some of these pigment colorations out of season. Hardly ever with oakleaf hydrangeas. More often with mopheads. And some times with roses' new leaves.
What happens is that only oakleaf hydrangeas tend to easily develop and display the necessary red/purplish/orange pigments at the end of the season although, they need some sun to produce a good show. I have several oakleaf hydrangeas and the ones that get more sun produce a better show than the ones in more shaded locations.
If the shrub produces a lot of carotenoid pigments, you will see the medium yellows and-or oranges like in Little Quickfire. If your shrub produces a lot of anthocyanin pigments, you get the reds/purples/etc typically seen in Oakleaf Hydrangea and sometimes in a few other macrophyllas. You tend to see them in the Fall because the chlorophyll usually overpowers the carotenoid pigments in Spring-Summer; and the anthocyanin pigments get produced in the Fall when there are smaller amounts of chlorophyll.
Leaf spots are common at the same time of the year but a single leaf spot usually does not cover a whole large leaf. Instead you get several spots like so:
https://cherylyfisher.files.wordpress.c ... f-spot.jpg
Leaf spots tend to have purple/black/brown/yellow/etc. colors. These lesions are due to fungal infections, namely cercospora leaf spot.
Regarding the change of location for the plant in the picture, I agree. Hydrangeas do not tolerate being in rock mulch during the hot season. Either switch to hardwood mulch, pine needles/etc or transplant it. If it has wilting fits, you may want to temporarily put it in a pot and then plant it when cooler temperatures arrive. Or transplant it now but keep and eye on its almost daily watering needs & drooping wilting episodes. Watch out if you go on vacation as this is the so-called "dry season" in the Pacific Northwest.
Does this help? Luis
PS - did you know that the shrub in the picture is not Endless Summer Hydrangea? ES has now been renamed. The plant is now called The Original and the collection/group/series has been named Endless Summer. I know, I know.... I am only the messenger so I do not know why they did that. The Original is a mophead hydrangea. The one in your picture a lacecap hydrangea.... possibly Twist N Shout if it is part of the Endless Summer Series.
FYI - while talking about the ES Series, there will be a new addition to the ES Series next year (2019), a mophead with the name Summer Crush and raspberry-colored blooms.