Advice please

I transplanted some plug plants -Impatiens New Guinea Harmony into 9 cm pots using a general multipurpose compost. I placed them in a cold frame.
These were very healthy plants with green leaves. However after a few days the end of the leaves have gone a purpley/brown.
Can anyone suggest what the problem is. have I overwatered them or have they got too cold ?

 

It's probably still too cold for them in a cold frame.
They were brought up in totally ideal growing conditions, being plug plants, so need to harden off a bit as the nights especially can still be pretty cool.

 

Thanks Pete -maybe need to get my little greenhouse heater out and plugged in !