(PAL) The video signal format used in the UK [where else?].
PAL uses Amplitude Modulation for the video information, and Frequency Modulation for the audio information. The phase of the colour subcarrier is reversed on alternate lines which (together with the use of a delay line) allows the receiver to cancel any phase errors introduced in the path between the studio and the end-user's receiver. Such phase errors are quite common and would cause the displayed colours to shift in hue.
The US equivalent, NTSC, does not have this feature and thus requires a user control to correct for transmission phase errors, hence the mis-expansion "Never Twice the Same Colour".