I'm no botanist, but I do know that many plants come up purple from reseeding--columbine and phlox jump immediately to mind.

Don’t allow them to seed.

The botanical name - Digitalis purpurea - should provide the clue. The purple-pink-mauve colored flowers are the genetic foundation of this plant. The white forms are a genetic aberration. If allowed to self seed, the majority of the new plants will default to purple or pink.

A recent episode of Gardeners' World had a segment on foxgloves and apparently one can determine the flower colors of new seedlings by examining the lower leaves. Those that will bloom purple or pink have a reddish tint to the petioles and base of the leaf rib and those without any reddish tint should bloom white. I have not verified this myself, although foxgloves have naturalized here and appear everywhere, but also found this statement confirmed by Gardens Illustrated.

It's worth further investigation anyway if you want only white flowers. There is no other way to insure only white flowers unless you replant annually.

I know that with Dahlias the light colored ones (white or yellow) have a lighter shade of green in their foliage, pink or purple ones tend to have reddish veins or stalks. same with snap dragon seedlings

This reminds me of my coneflowers. 2 years ago I planted the orange/red variety. It was 10 feet away from my purple species. last year I got an bunch of seedlings under the orange plant. But all of them bloom purple flowers. But I love them all the same.