That's one of the reasons now with 2 way switching for a light for example it is mandatory to use the correct cable colours R,W,B 3 core TPS.
Green or black are not be used for phases and also not to be sleeved with a phase colour as Mike ( Trumpy ) already mentioned.
POCO's seem to have their own rules and it's very bad to see the black, non colour coded cables in pillars used for phases and green phase wires used as droppers in transformer cubicles.
My penny worth on this topic
Raymond
