Why are my petunias losing color? They were bright purple when I bought them and now they are...

They were bright purple when I bought them and now they are turning white. Am I overwatering them? Not the correct nutrients contained by my soil? I used miracle grow about a week ago and they are growing. Please help.
Answers:    Mine did like peas in a pod this year when we had too much rain in a minute they have regained their bright purple color. Some of the leaves where on earth starting to turn yellow also but all that have gone away now with smaller number moisture.
Marvin is correct. Tp try and promote the hybrid color try increasing the iron and magneseum in the soil. Acidity will attempt to bring out dark colors surrounded by plants they are hybreds. And the original dominant gene is taking over.