Or at least what works for me.
When it comes to painting in cold weather unless you have a heated shop painting large objects like an entire body is probably not a good idea. However, you can paint moderate sized panels if you take some precautions first.
To begin with the temperature of the panel needs to be as warm as possible (within reason of course). I use heat lamps placed about 3 feet away (on both sides of the panel) and leave them on long enough to warm the metal to at least 75 degrees. You can see how I used these on my gas tank below.
Because modern paints need a temperature of at least 55-60 degrees in order for proper cross linking to take place it is important to not only preheat the metal but continue to use the heat lamps during and after painting. I leave them on for at least 4 hours after the last coat but 6 hours wouldn't hurt. Here's the finished product. One deuce gas tank painted 1964 Ford "Candy Apple Red". Its not really a candy but that's the name Ford gave it. In this case I used Martin Senour's Crossfire single stage urethane over a two part etching filler. Two mist (tack) coats followed by a medium wet coat and then a wet coat with 15 minutes between all coats. Outside temp 30 degrees, shop temp 50 degrees, metal temp at least 75-80 degrees.
The results were outstanding when you consider it was not painted in a booth and the conditions weren't optimum. I've used this technique in even colder weather and it seems to work just fine. Painted the cab on my pickup using heat lamps and PPG Omni single stage about 5 years ago and it still looks perfect.