Wiring colours

[Davey101]

I did a PIR the other day where the 2 way switch had been wired in 3 core flex (ie brown, blue and green/ yellow) and the G/Y was used as one of the lives.

Called the NIC and their comment was it was ok as long as sleved Brown at both ends, with the qualifier that being flex the wire was insulated along its whole length (as opposed to the bare earth in a T&E cable) and the house had RCD protection.
Also in this case the cable could not be removed without major disruption and rework.

Made a big note on the PIR and stuck a lable on the CU to warn future electricians.

Why did the house have to have RCD protection because a conductor is sleeved a different colour?