Indiana has simply not had many days like that in 1996. Consequently, many fields are an ugly yellow-green rather than the darker green that we would prefer to see. 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. Single white plants in a field are usually genetic mutants.