PowerSwitch

Not applicable to standard AC installations, but the colours for DC have changed/are about to be changed.

2 wire DC, floating with neither side earthed.
RED=POSITIVE, WHITE=NEGATIVE.

2 wire DC negative earthed
RED=POSITIVE, BLUE=NEUTRAL/NEGATIVE.

2 wire DC positive earthed
BLUE=POSITIVE/NEUTRAL, WHITE=NEGATIVE.

3 wire DC center earthed.
RED=POSITIVE OUTER, BLUE=CENTER WIRE/NEUTRAL, WHITE=NEGATIVE OUTER.

The old colours were brown for what is now red and grey for wahat is now white. Neutral was, and remains blue.
Protective earth was, and remains green/yellow.

The old OLD colours were
2 wire DC floating with neither side earthed.
RED=POSITIVE, BLACK=NEGATIVE.

3 WIRE DC, center earthed.
POSITIVE OUTER=RED
CENTER WIRE=BLACK (sometimes white unofficialy to avoid confusion with AC neutral)
NEGATIVE OUTER=BLUE.

2 WIRE DC DERIVED FROM A 3 WIRE SYSTEM.
LIVE=RED (wether derived from positive or negative outer)
NEUTRAL=BLACK
Sometimes, the live wire of a 2 wire system that was derived from the negative outer of a 3 wire DC system used red wire, but marked with blue sleeving to denote this.

Protective or safety earth was and remains green/yellow.

I am old enough to remember not just plain green for earth, but also BLACK earth wires.

A functional earth, as distinct from a protective earth is now to be pink, was previously cream/off white.

Maybe this is to do with US trade where black was, or is, the live wire in flex.

Possibly, yes.
In the USA white is used for neutral and black for live or phase.

This was one reason for discontinuing use of black for neutral in the UK.

USA colours.
For lower voltage systems such as 120/208 or 120/240 volt circuits.
White for neutral
Black for live
Red, for the second live wire in single phase 3 wire 120/240 volt systems.
Blue for the third phase in 3 phase 4 wire systems at 120/208 volts.
On a 3 phase, 4 wire delta system, the "high phase" which is 208 volts to neutral should be orange, and the other two phases that are 120 volts to ground should be black and red, or sometimes black and blue.

For higher voltage systems such as 3 phase, 4 wire at 277/480 volts, and higher.
Neutral, gray.
The three phases being brown, orange, and yellow.

Regulations permit the neutral to be white or gray on any voltage system, but white for 120/208/240 volt systems and gray for higher voltages is an accepted convention.

Regulations permit the live or phase conductors to be any colour other than white, gray, or green. The colours given above are the established convention and not a requirement in most cases.