Soil Conditions

• The hydrangea macrophylla, the most common of garden variety hydrangeas, develops its blue color when the soil has adequate aluminum. The pH of the soil must be 5.5 or lower for blue. Anything higher will produce hydrangeas of varying pinks.
  
  

Adaptive Radiation

• Flower color transitions happen when the types of anthocyanin pigments change production or because of their complete loss, according to a study on adaptive radiations by Scott A. Hodges and Nathan J. Derieg published on National Academy of Sciences website.
  
  

Gene Mutation

• The study by Hodges and Derieg talks of 34 different genes involved in the production of the flowers' color pigments. Another theory, written by Brian Thomas, M.S., for the Institute for Creation Research, suggests there are many places in the genetic makeup of the flower that give itself to genetic mutation, which means that at certain points the needed pigment gene isn't there or has multiplied abnormally.