Purpling of young seedlings is very common while they are young and still developing the roots needed for effective nutrient use. They outgrow it quickly as they mature. It is a common question and can be a bit shocking when you first see it but while it can be a cause for concern in well established plants - it is a sign of low phos - it is no problem in seedlings.

Dave

Thanks This is the first time with petunias from seed. I was a little panicked

My tiny seedlings are purple so this was good to learn. Dave - how many weeks from starting them might I see some real top growth on the petunias?