Answer: The most common reason for the buds failing to open fully is lack of adequate sunshine. Your plant simply doesn't have the energy required to open its flowers. Try giving it morning sun and afternoon shade - or filtered afternoon sun. 3-4 hours of direct morning sunshine should be enough to make those flower buds open.
White hydrangeas will remain white; if you have a blue or pink flowering hydrangea you can keep it blue or pink or change a blue to a pink or a pink to a blue by adjusting the soil's pH (adding sulphur or lime). Since yours in normally white, it will remain white regardless of the soil pH.
Best wishes with your garden!
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