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I'm fairly certain that white hydrangea's stay white whatever the PH.
You could use a lime free compost to be on the safe side.
You should always use ericaceous compost when potting on Hydrangeas anyway. This will keep the colour true.
Your plant should stay white but if you want to be sure you can water it with sequestrene in rain water if you live in a hard water(limey) area. If you want a blue flowered plant then water with hydrangea colourant and the plant will turn blue.
Mark, colourant only works on blue Hydrangea which have turned pink through being in limey soil/compost. It will not turn a white Hydrangea blue.
Sounds like my white flowers will be safe. I have some ericaceous compost, thanks for the replies.