Don't get hung up on colours, they aren't used to identify wires.
Either trace it back from the light cluster or track it on a wiring diagram using the numbered connectors and pins
on my 09 berlingo van. the wire on the:
left side is yellow,
right side is Brown.
when i looked on the plugg for the back lights.
the reversing light cable is the bottom cable. and the Third cable from the botom is big ground cable (yellow/green).
use a testligt to find the correct.