Hydrangea color

We started some hydrangeas a couple years ago and they are all growing well or better.
However, their colors are unstable!!
There are 6 plants: Annabelle, Pia, and Lady-in-Red along a southern fence, and Blushing Bride, Endless Summer, and Izo No Hana(H. macrophylla} along a northern fence. The first years they kept their expected colors (the ones we started with, in parentheses).
This year, Annabelle remains white (white), Pia is pink w/ yellow(dark pink), Lady-in-Red is blue(pink/red), Blushing Bride continues its varying pale colors but darker, Endless Summer is pink/lilac(blue), and Izo No Hana is pink/red(deep blue). In short, the whitest have kept their coloring, the reds have trended blue and the blues have become red/pink.
Any suggestions for treatments to bring back their original colors?

Annabelle won't change colors with soil pH so they'll always be the same. Your others will all change according to the soil pH, although Blushing Bride will stay a very pale color so you won't see the effect as much. The others will be pink in alkaline soil and blue in acidic soil. The fact that some of yours have gone to blue and others have gone to pink tells me that you have a different pH in different parts of your garden. By any chance, are the ones that are now pink planted near something concrete (foundation, sidewalk, driveway, etc)? Lime can sometimes leach out of the cement and raise the pH in those areas which would explain why those hydrangeas are pink and others blue. Or did you add any amendments to one area or the other that could have changed the soil pH? Otherwise it just means different areas of your garden happen to have a different pH.

If you want them all their original colors, you could move them--put the ones that you want to be blue in the area of your garden where you're currently seeing blue, and put the ones that you want to be pink in the area where you're currently seeing pink. The other option is to amend your soil--where you want pink flowers, you'll need to raise the pH using lime, and where you want blue you'll need to lower the pH using sulfur or aluminum sulfate. The catch with amending is you'll need to keep doing it each year or else the soil will tend to drift back to its natural pH. For Pia which used to be darker pink and is now lighter, if you raise the pH a little bit it may go back to being darker pink.

We tested the pH with a kit before we planted them and it was about 6.5-7.5 in both areas. All of them were planted into highly amended soil as the yard's soil is very much a fine sand for the first 3-4 feet, with layers of clayey and gravelly subsoil below that in some areas. I can't say about all areas because there used to be ravines and such in the area that have been filled and leveled. Do you know how far down Hydrangea roots feed from?

Those pH kits aren't necessarily super accurate, and also pH can shift over time--could be some of the amendments you'd added initially have decomposed and the pH as changed over time. Even if it's stayed the same though 6.5 is slightly acidic and 7.5 is slightly alkaline it wouldn't be surprising that you'd see a mix of blue and pink (and likely some lavenderish ones in the middle). If you want specific ones to be blue or pink, you'll still have to do what I suggested and add lime to make pink and sulfur or aluminum sulfate to make blue. Hopefully you don't have ones that you want to be pink sitting right next to ones that you want to be blue--it'll make it easier if you don't try to amend soil around one plant to be more acidic and the soil around the plant right next to it to be more alkaline.