I've been given a variable heat soldering iron and was wondering what the best/safest temperature was to operate on small Arduino boards, pins, etc. It has 3 colour zones:
What sort of solder are you going to use.
For Lead / tin solder I would use orange or yellow. Lead free solder will have to be hotter.
This can sounds a little bit backward. with a hot tip you can make faster solder joints and do not heating up the board round the joint.
I say minimum 350 °C
With leadfree solder we have 400°C on the tip
With a cooler tip, it takes longer time and the heat spreads over a larger area.
Pelle
Its a 60/40 lead/tin.
The 340 ~ 350oC (orange) is ~ half way around on the pot/dial, so looks a good place to start.
I see what you mean about a hotter tip creating faster solder joints also
Thanks for the help 
With lead solder, I normally run around 400 to 425C when soldering leaded components and connectors. As Pelleplutt says, get in there, get it soldered before the heat spreads.
It does depend on what you are soldering, though. I often run cooler when soldering surface mount, closer to 350C.
For lead-free, it depends very much on the exact formulation of solder. I have to recalibrate when I try a different mix, sometimes I find 5C variation is enough to not work well.
I think I have lead-free solder (the reel doesn't actually say) and I am running at 425 C.
My policy is "get in, get it over with, and get out". That is, do it quickly.
"get in, get it over with, and get out"
The above statement could correlate to a few scenarios, haha!
I think I've got the gist of it now, thanks
Just christened the iron by accidentally burning my finger! 
On the plus, I managed to successfully rescue an Arduino Nano that had its pins soldered to a pcb board.