Indiana has simply not had many days like that so far in the 2000 growing season. Warm, sunny days will correct this. Purpling results from the accumulation of a purple pigment called anthocyanin . Whether or not a corn plant produces anthocyanin is determined by the hybrid's genetics. Some hybrids contain more 'purpling' genes than others.
Purple corn is caused by one of two factors.
The first factor is simply a genetic response to cool nights following bright, sunny days. Warmer weather will cause the purpling to slowly disappear. The second factor is restricted root development, coupled with an abundance of plant sugars produced by photosynthesis that triggers the purpling. Remember that the effects of early season damage to the seed or root system can be magnified when corn is already developing slowly due to cool, cloudy weather. Single white plants in a field are usually genetic mutants.