Your site states that a white hydrangea will always remain white. My pure white hydrangea has now turned a dirty pink tinge around the edge of the petals. The soil is medium ph but is on the edge of a veggie patch which had well rotted horse manure dug into it last year - could this be the culprit?
I've had a white one for a few years now, and mine has done the same. The thing with hydrangeas is that they need a specific type of PH soil. That's why you when you have either a pink or a blue, it depends on what ph your soil is.