I will be surprised if either change to blue. I tried that stuff for two seasons and mine looked horrid, no blooms about killed them. I have to say I am extremely disappointed in these cultivars. TOTAL misnomer "endless blooms" how about a few blooms every four years!!

You can always try but if the root systems meet, you will get colors approaching to the soil pH in that spot on that side of both shrubs. Need to be careful to amend the soil of the blue one and not spill amendments into the pink's root systems area. I was not clear which color is "normal" for your area. Be advised that there are some mopheads that resist turning blue. Using containers, this may be easier. Careful when applying amendments containing sulphur to turn them blue; if you exceed the recommended dosages by a lot, you may burn the roots and cause blooming problems like in the post above.

How successful you may be at this will depend on what your soil's pH is currently. There is a limit on how much impact "hydrangea bluing" (aluminum sulfate) will have if your soil tends to be alkaline to start. And as Luis notes, if the plants are close together, it is much tricker to apply without cross contamination.

To address the first response, Endless Summer does have a shaky reputation for performance in colder climates, those that it was developed to thrive in. Cold winters often knock the plant back to the roots, eliminating any potential for flowering on the old growth and often seriously delaying or even hindering its ability to flower on the new growth. If you live in colder zone (6 or below) I would suggest winter protection, just like one would need for any other non-remontant selection.

In my climate, ES is a very reliable performer, as are all other bigleaf hydrangeas. And my soil conditions naturally lead to a very blue flower coloration. I have no idea how well the other, "free flowering" or remontant selections perform, as that attribute is just not necessary here and I don't grow any of them. Perhaps someone else in colder zones can comment.

Adding info on your location will help here since ES isn't totally reliable at blooming in the cold end of its growing zone, and we can perhaps make guesses about your soil pH based on your location (though having a soil test will be better so you have an idea of what and how much to add to the shrub you want a color change on.)

To get your location to post automatically with your name:

Go to Your Houzz in the upper right of every page, click Edit Profile, and on the left side click advanced settings. Well down the advanced settings page is a blank labeled Climate Zone for Garden Forums along with a link to find your zone.

If you add info on your state or the nearest large city you will get even better information since zone only relates to average coldest winter temperatures.

Then return to the top of the page and click Done Editing.

Ok my zone is 7a in northern NJ. Summers are pretty hot and brutal. These hydrangeas get around 4 hours of morning sun and early afternoon sun. I think the soil is naturally acidic because one of the hydrangea I bought as pink blooms are now turning purplish. Also its right in front of the foundation as you can see in the pictures.