I contacted PPG and asked them the technical reason for not spraying Concept below 60 degrees.
Here is the answer I got from them.
Thank you for your interest in PPG Refinish products. My concern is the low temperatures you are describing. Once you go below 55 degrees the paint stops curing, it doesn't restart if the temp. goes back up. You must maintain at least 55 degrees for a minimum of 24 hrs. Once the paint goes below 55 it permanently loses its ability to reach a full cure.