I too would have determined the soil pH (which you already did) and what levels of aluminum the potting mix has. Normally, a soil pH of 5 would have turned most macrophylla blooms blue or purple, not faint/light pink as this so this tells me that the amount of aluminum in the potting mix is zero or very small. It is the aluminum what lets the sepals turn blue/purple at acidic levels.

Feel free to amend the soil frequently to maintain the soil pH at 5.0 (or thereabouts) but do not use aluminum sulfate or the color will change to a shade of blue.

As far as what amendments to use, you can choose from whatever you use now, garden sulphur, greensand, iron sulfate or ammonium sulfate. Iron-chelated liquid compounds can also be used but check if they have aluminum and choose those without or with the lowest levels of aluminum.

Note that you will need to do this monitoring forever in order to control the soil ph. But at least your shrub is in a pot wherre the soil pH is easier to control (compared to plants in the ground).

Since soil pH scale is logarithmic, numeric displays in readers are more useful than kits which display colors when you mix soil, water and a pill. So continue using those units.

When replacing the potting mix in the future, do not choose potting mix for acid loving plants.

Luis

Thanks- yes I didn't understand why it wasn't blue. I do intend putting these cuttings in the garden when they are bigger so it's probably not feasible to try & keep this colour.
But I'll dig a big hole & fill it up with pH 5 soil, I wonder if a mature plant would survive in the same soil the cuttings are in, which is just peat moss and perlite?
Do you mean that potting mix for acid loving plants is high in aluminium?

Once you put the 4 cuttings in the ground, then the soil pH of the ground soil will determine the color and the intensity of the color.

Adding soil (whose pH is 5) in the ground hole will turn the blooms blue if there is aluminum in that soil you will be adding. If there is none, the blooms will remain a shade of pink. The soil in the ground usually -but not always- contains enough levels of naturally occuring aluminum to turn blooms blue if you acidify the soil. But if the ground soil or if the potting mix is not acidic, the plant will not absorb aluminum.

Note that once the roots grow from the soil with pH 5 and get into the garden soil, the garden soil's pH will make the bloom colors change. This will not happen in one year though but over several ones.

Potting mix for acidic plants is not necessarily "high" in aluminum; it may have a lot, some or none like yours. If the product says "for acid loving pants", there may be some aluminum added. For example, if you see aluminum sulfate in the list fo ingredients, you would have aluminum in that potting mix. This is one way that aluminum would be added to potting mix. So, to keep the blooms pink in your current pot, look at the labels to determine of there is aluminum in the ingredients (beware - some labels are pretty bad at listing these minor minerals though). Better yet, buy the same brand you are suing now (if you know which one it is).

Peat moss may be the ingredient that is giving you the pH of 5 but beware of using too much of it because if it dries out, it can begin to repel water and start another bucket full of problems. I have not heard of anyone growing hydrangeas on the ground with perlite & peat moss so I cannot comment.

Thanks - it's complicated! I didn't use potting mix at all, to strike the cuttings I just mixed perlite & peat moss and they are still in that.
I'm looking forward to seeing what colour I end up with - they are all beautiful so maybe I shouldn't fight nature!

Enjoy!