Answer: Roses are often grafted onto a different (more hardy) rootstock. It sometimes happens that growth from below the graft is so vigorous that it overtakes the grafted part. What you are seeing are probably the flowers from the rootstock rather than the grafted white rose. You can examine the shrub to see if anything is still growing from above the graft. If so, cut off anything that is coming from below the graft and allow only the grafted part of your rose bush grow. You should see white instead of pink flowers.
Good luck with your roses!
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