Answer: In general, the pink or red flowers do fade and change color as they mature on the plant and this can sometimes provide a pleasing, multi colored effect. When the flowers are past their prime and go to seed and dry out, they typically bleach out and turn white with age. Regular deadheading would prevent that from being seen in the garden. Sometimes too the weather can cause poor color quality, as when night temperatures are above about 70 degrees.
However, based on your description it sounds as though the plants may be self seeding and reverting to the species color -- which would account for all white blooms from the beginning of the season onward. Regular deadheading would prevent self seeding. Also be sure you are buying plants that are vegetatively propagated (division or cutting) to be true to name as seedlings can often show a lot of variation in color.
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