Have they flowered before? How old are they? Could they have been grafted and are flowering from below the graft?

Mislabelling?

I'm curious about this one - how long have you had them and have they always flowered white in the past? I've not got an answer, but have observed over the years colour changes in various plant flowers, sometimes a permanent change, occasionally a temporary one. Permanent changes are usually because the plant is a new introduction or hybrid, and the genetic information appears to be weak, so that it soon loses its characteristics and reverts to one of its parents. Temporary changes (one year only) I'm not sure about at all, but am interested as to why this happens.

Bamboo, what you're saying chimes with what a friend was telling me the other day. She showed me a camellia that's been changing colour steadily over several years. It had striped red/white flowers and was going back to - (was it red or white? My memory!). Her daughter-in-law was a geneticist before having her family and had explained all about reversion to my friend. Who didn't really take it in & couldn't explain it to me. I tried to look it up for this question and gave up, but "reversion" could be the answer.

Reversion is the answer, Beattie, well, for your query anyway (regarding the Camellia).