That suggests that your roots are now growing into the ground soil and out of the potting mix. The original potting mix had the soil pH and the minerals balanced by the wholesaler to produce that shade of color. The only way to continue to get blooms of that color would be to somehow determine what soil pH/mineral levels the wholesaler had in the potting mix, etc and making the garden soil the same.

It sounds like your garden's soil pH is more alkaline than the potting mix and the blooms are now leaning to a pinkish shade. If your soil is normally alkaline, try adding some garden Sulphur or aluminum sulfate per label directions to acidify it and see what color you get. If it is acidic, add some garden lime (per label directions) instead. This is usually easier done with a potted plant so do not be surprised if you "overshoot" or end with parts of the plant one color and others a slightly different color. Write good notes on a wall calendar indicating when you amended the soilk and by how much so you can tweak (add/subtract) if that was not enough or if that was too much.

I think I once had your problem. I was traveling in SC and saw an awesome looking purple hydrangea at a grocery store. I think if I were to see and buy such a specimen, I would immediately get a soil pH test as well as a mineral test to check for nitrogen/phosphorus/pottasium levels because to me, those blooms were just awesome! If that did not work, I would see trying to get a lifetime subscription to some of that potting mix somehow! Hee hee hee!

just rescued 3 of the TS from Wal-Mart for $7.00 each. Amazing what water will do-they perked right up