It is probably a seedling from the original plant, they do come in pinks and whites as well as the blue.

Do you mean it will revert back to its blue colour, or will it stay white now ?
And, why haven't the other plants done this ?

Louise, if it is a seedling, it is a separate plant, and will stay white. It may be a "bud sport" from one of the other tubers, in which case, it stands about a 2% chance of reverting back to blue. Another possibility is adjacent buds fom the mother tuber producing offsets with parti-colored blue and white flowers, later! Of the three possibilities, that the white one is a seedling is the likeliest.

Thank you both, bit miffed though because i really like the blue ones !
This happens quite a lot then, yes ?

If pollinated with your neighbor's white anemone, about 50% of the seedlings will be white. If self pollinated, about 25%. Bud sports are about a one in ten thousand sort of thing--plant breeders love 'em!

There aren't any white Anemones in my garden and the neighbours here don't have flower gardens - theirs are grass only, so no white ones to pollinate them.