It's a genetic anomaly due to unstable genes for the color purple. Is it normal? Not really, but it happens occasionally. It goes back to the breeder. Enjoy them for the few days they last.

Either that or you have two plants growing together. We often used to get seedlings growing in the same place as the parent plant. The seedlings were always different colour to the parent.

I agree most likely two plants, especially as most of the white ones are together on one side of the clump. If you investigate the origin of the flower stems you should be able to see whether the white ones are all coming from the same place. Its a pretty effect anyway!

I sometimes see that in my garden - two seedlings growing very close together and they look like one plant with different colour flowers. It is difficult to separate them so I leave them alone.

I agree and it's a nice color combination