Green guard... 3 wing tanks of the Airbus. Outer, towards wing tip. Inner, between outer and wing root. And center.
The outer tank. is always the coldest fuel tank hence transferring it to the warmer inner tank. Warms the -40 fuel up by mixing with inner tank fuel which is much warmer. From memory, somewhere in the mid -20s for inner tank when the outer hits -40.
For low temp in Trim tank, transfer to the centre tank.
If youre not increasing speed/ finding warmer air then transfer is your only option. At my carrier, first option considered is transfer. We operate on track systems so descending to lower level is generally not an option due to the time factor involved with ATC.