Why is the battery icon red and not green when charging?

I received a messsage that the battery was low. When I picked up the phone I found that the torch was on. Not knowing how to switch off the torch (i do now!), I switched the phone off, which naturally extinguished thr torch. I then plugged in the phone to my charger, and I got the "ping", but instead of showing the battery icon in green, it was red. Why, and what is the cure?