[emeritus]

The only plug with coloured sleeved pins i have seen in the flesh was a 5A 3 pin plug of German manufacture fitted to a standard lamp in a hotel room.

You can only get a patent for something which only differs from what is known by its colour under special circumstances. AFAIR the key case was one for a coloured squash ball, where the colour (blue I think) was shown to be more visible than the known type and so resulted in a real technical effect.

It seems that the public are actively discouraged from fitting their own plugs these days. In the current CPC catalogues, the mains leads supplied with bare wire ends all have a warning in red ink that they are "Only for use by qualified personnel", no doubt to cover themselves and/or to comply with legislation.