What proof do you want?
Proof of the UK wiring colour scheme can be found within BS7671 but that can only be obtained by paying for it or getting a dodgy copy.
You can have a look at the wiki if you wat, but as that can be modified by anyone would that actually be proof for you?
https://en.wikipedia.org/wiki/Electrical_wiring_in_the_United_Kingdom#Wiring_colours
How to prove that there is no wiring colour standard for within equipment would be nigh on impossible due to the lack of a standard on it.
Although BSEN60335 may be a place to look, yet I have not seen anything mentioned on the subject in it.