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.
They take a while to respond to the new conditions. But don't use tap water to give them a drink if you are in an area with hard water - this might add a pink tint. Use rainwater and they should keep their original colour.
Pink and blue hydrangeas will change colour in response to the soil conditions but white hydrangeas will not be affected and will stay white whatever you do. Most white ones will naturally shade through to pink as the flowers age so, if you want to keep them looking white, just deadhead when they start to turn pink.
Not sure they will stay white, I dig my whites out after a couple of years,they first turn to pale grey then a dirty pale grey/ blue, then they're out.
i have just one left now that had stayed white, but my soil is very acid,