from what I've learned from speaking with our local garden center staff, and googling.. the purple leaves could be from the aluminum choking out the phosphorus in the soil. If you are using aluminum sulfate to get the soil ph you have, maybe consider switching to sulfur? I'm not a chemist nor an expert so still do your own research but maybe this can help give you direction. good luck!

Likely it's because they are getting a lot of sun exposure, that they were probably not getting before purchase. For instance the Bailey nursery introduction sold using the Twist 'n' Shout trademark usually has purplish leaves in sunny positions.

I did not use our clay soil but mixed compost with topsoil.

In future don't do this when planting long term subjects like shrubs unless you are replacing the soil over a large area such as a bed of some extent. Plant instead in loosened existing soil, without mixing in amendments such as compost etc. This is because amending of small areas such as individual planting holes often has undesirable effects on how water enters and exits the amended zone immediately around the newly installed plant.

Can you post a picture and indicate in what city/state are you in Zone 8? Can you also identify the type and variety using the plant label? I am not sure which type of hydrangea is it: a macrophylla, an oakleaf, a paniculata, etc? Thanks.

https://plantaddicts.com/types-of-hydrangeas/

I've found that purple leaf discoloration is very common with newly purchased bigleaf hydrangeas. This is due to cercospora, a fungal disease. Plants are all bunched together in the nursery and they overhead water , both things contribute to fungal disease. Typically, it does not affect the vitality of the plant so can just watch it if you want to. Do a good fall cleanup and may not be an issue next year. You can also treat with anti-fungal spray if you wish.

That is primarily spotting

https://www.bing.com/images/search?q=cercospora+leaf+spot+on+hydrangea&FORM=HDRSC2

rather than a purple tinting without spots