Try saying that 3 times fast! This is one of the Endless Summer hydrangeas that I bought about 3 years ago. It bloomed so well this season and I even cut a bunch of blooms earlier in the summer to put in the house. At that time of year, they were pink. But as summer wanes, the blooms start to change. They take on a sort of dried look and I think it adds some interest to a late season garden.
As I mentioned, the blooms were pink. I tried to change the color to blue by acidifying the soil using a garden sulfur product by Espoma (it can be found in any big box store or garden center). I think I did not apply enough or with enough frequency. I think maybe the latter because I water every day so anything in the soil washes out fairly quickly. I'll try again next year.
On to the subject of pruning. Here's what I do: Nothing. Endless Summer blooms on old wood and on new wood. Old wood refers to the stems that have been on the plant since last summer. New wood refers to the stems that developed this season and that continue to produce buds through the fall. If you cut back those, you know what happens. No buds this year equals no flowers next year. So why cut back any of it? The only stems you really need to trim are any dead ones.
Finally, fertilizers. I only use a slow release fertilizer at the beginning of the season. If you fertilize too much, you'll get lots of pretty foliage but few blooms. So as far as fertilizers go, less is more.
Hydrangeas make a great container plant. You just need to make sure you have a large enough pot because it will definitely multiply in size every year.
Bookshelf: Container Gardening Books