Difficult to see how there can ever be a "must".
1. It's "only" (tricky to phrase, as they are important) the wiring regs that require you to use the new colours, and they are not a legal requirement, nor are they mandated by the Building Regulations.
2. What is a legal requirement is that "reasonable provision shall be made in the design and installation of electrical installations in order to protect persons operating, maintaining or altering the installations from fire or injury". When it comes to extending, altering or maintaining an existing red/black installation I think it's more reasonable, and safer, to carry on using red/black than arbitrarily deciding to mix the colours.
1. It's "only" (tricky to phrase, as they are important) the wiring regs that require you to use the new colours, and they are not a legal requirement, nor are they mandated by the Building Regulations.
2. What is a legal requirement is that "reasonable provision shall be made in the design and installation of electrical installations in order to protect persons operating, maintaining or altering the installations from fire or injury". When it comes to extending, altering or maintaining an existing red/black installation I think it's more reasonable, and safer, to carry on using red/black than arbitrarily deciding to mix the colours.
