[anyfoo]

Good explanation, though "phase-modulated" is not the whole picture (hah), it's most correctly quadrature amplitude modulated, which is basically a fancy way of saying that there is a modulation in phase and amplitude.

The two signals in the quadrature amplitude modulation are the two color difference signals. While that is the intent and also usually how encoders and decoders are actually implemented, most elegantly the math works out to a different interpretation as well: You can see the phase as the hue (i.e. what color), and the amplitude as the saturation (i.e. how much color).

That alternative interpretation works extremely well to explain the whole scheme without having to explain how and why quadrature amplitude modulation works (which needs a lot of math).