Hi Dominic, different throttle and controller manufacturers use different wire colors for the functions of their products so we can not figure this out without seeing wiring directions for the two parts that are being connected together.
Do you have or can you get wiring directions for the new throttle and for the brushless controller so we could have a look at them?
To do this unplug the motor from the controller. Then take a very small screwdriver and one at a time push the locking tab of the terminal pin flat with the pin and gently pull the pins out of the white plastic connector. Then restore the original positions of the terminal locking tabs using the same screwdriver and push the terminal pins back into the plastic connector in the opposite positions that they were in when they were pulled out. I recommend marking the position of either the red or black wire with a permanent marker or tape so their original positions are not forgotten after the terminals are removed from the connector.
Please let us know how it goes.